When I visit the States I see a lot of pasta and spaghetti in the supermarkets. I wonder which is the typical seasoning (sauce or similar) used by Americans when they cook spaghetti. Of course, when they don't cook Italian-style.
The most common preparation is tomato sauce and parmesan cheese.
Of course, some Americans will tell you that the only truly American way to eat spaghetti is to top it with chili, then shredded cheddar cheese, then chopped onions, then red kidney beans. It's hard to fully express the majesty of this dish, and should only be enjoyed on special occasions, like the Super Bowl.
In Canada (which I consider to be basically "American"), it's also fairly common to add or replace the tomato sauce with pesto sauce. Soft goat cheese is another common additive. Perhaps these sound "Italian" but I'd consider an Italian pasta to be something more like a bolognese, carbonara, alfredo, or even the trusty old butter/olive oil and parmesan cheese. You'll almost never see tomato sauce combined with pesto sauce in an authentic Italian restaurant (or in Italy), but it's common in American home cooking.
I might refer you to this video, where Malcolm Gladwell talks (a little bit) about spaghetti, and how Americans prefer to eat it.
Apparently, companies doing market research found that Americans say they want "real Italian pasta sauce," which is somewhat thinner than "American style," but actually prefer eating the chunkier, heartier sauces now common on supermarket shelves.
Besides what @Aaronut mentioned with Cincinnati style chili, if we're talking pasta in general, and not just spaghetti, I'd say the "American" pasta dish is macaroni and cheese -- from a box.
Although there's regional variations (eg, lobster in New England), it's found in a much wider area than (N)-way chili (3: spaghetti,chili,cheese; 4: add onions; 5: add beans)
Also popular is "macaroni salad", at least along the east coast from at least Pennsylvania to Georgia.
My mom's second-generation Italian-American, so I grew up with carbonara / pesto / oil and garlic / butter and parmesan / crab / tuna / primavera / bechamal / etc (bascially, whatever was in season and/or cheap). My neighbors on the other hand (friends from high school, we take turns cooking), never cooked anything other than tomato sauce from a jar and/or mac and cheese (from a box). They were even surprised when I introduced them to chili over pasta (but their kids loved it).
By far, my most common pasta dish to cook is spaghetti in tomato sauce, straight outa my California Heritage Cookbook. Can't get more American than that, can you? ;-)
This tomato sauce is no marinara. It has ground beef, mushrooms, onions, garlic, and celery, and is otherwise made from a base of tomato sauce (29 oz) and tomato paste (6 oz), which gives it a particular consistency and flavor that I'm guessing is more American than Italian, since none of the Italian restaurants I've been to do bolognese in quite that way. The olive oil used for sauteeing the beef & mushrooms is first used to sautee the onions and garlic, which lends a fantastic aroma to the kitchen.
Baked spaghetti is another classic that my elementary school cafeteria loved to do. Take leftover spaghetti in meat sauce, liberally sprinkle cheddar and/or Monterey Jack cheese on top, and bake in a 350F oven for 30 min or so. For the full American educational institution experience, serve with an ice cream or similarly shaped scoop onto a styrofoam tray. :-)
Baked macaroni and cheese is the third I'd mention. It involves layers of large elbow macaroni and cheddar cheese, plus a dash of paprika in the sauce for flavoring (well OK, these days I use a bit more than a dash :-) ). I use the recipe from the Joy of Cooking. For the full-out American experience (not for the faint of heart!), substitute "American cheese" (can you even get this outside of the States?) and butter for the cheddar.
I think you're looking for New York Italian pasta.
The major influence on New York pasta was the unavailability of good quality pastas for quite some time.
The changed the ethos of the dishes from "It's all about the pasta" (the Italian way) to "It's all about the sauce" (the American way)
So what happened was the sauces became very heavy meat sauces (almost a chile con carne but without beans)
A typical recipe would be:
I Can Tomato sauce
1 Can Tomato Paste
1 lb Ground beef/pork/sausage meat (or all three)
Green and red Peppers,
and of course, the Ubiquitous "Italian Seasoning" which is a blend similar to Herbs Provencal, but much heavier on the oregano.
Brown meat and drain, Chop and saute veggies, Mix it all together, and simmer for an hour or more.
You'll end up with a pretty thick sauce that can almost be eaten as a Sloppy Joe.
Keep in mind, as with all Italian cooking, there is absolutely no such thing as an "Authentic" or Canonical recipe. Each person has their own version, and there have been bloodbaths in the streets over who's mother makes the better dish.
The history of american spaghetti and meat sauce. It's popularity in America can be traced back to a cookbook published in 1920 by an association of pasta manufacturers.
Per the mass market/tv ads - with red sauce of some sort.
Per I hate tomato sauce on pasta, not in my house.
Just cheese is one method - I prefer a sharp cheddar, but am happy enough with a pecorino romano that's more convenient (seems fine if purchased grated/powdered, kept in fridge) while the sharp cheddar needs to be grated onto the pasta. Most american supermarket "parmesan" might as well be sawdust.
As a gardener (and I suppose per one answer, living near Canada, though I would not think of it as particularly Canadian) straight pesto or pesto with additional cheese is another, not too far off the reservation method.
Branching out into "thought odd" (but very American) land, peanutbutter, peanutbutter and vinegar or peanutbutter and mustard (think of it as "simple peanut sauce" if you like that thought better.)
"Macaroni and cheese" is a widespread product as well, in many variations from horrid to excellent, but done traditionally it's far more work/time than simply grating cheddar on angelhair is. If you apply the same logic that makes red sauce default (sheer volume and lots of advertising) then the horrible (kraft powdered orange "stuff" and competitors in "convenience") rules this market, and probably equals or exceeds the red sauce pasta sales volume, actually. It's "easy food that's widely accepted by children." I personally prefer radiatori as a pasta for for it, and the non-packaged bake it in the oven method if going to the bother at all. Grating cheese on hot pasta usually wins.
Lasagna has an arguably more minor, yet significant, niche.
Egg noodles often show up simply buttered, or in soup.
The Oxford Companion to American Food and Drink (via googlebooks) suggests that Mac and Cheese is both of earlier provenance in the US and indeed still exceeding "with red sauce" in sales.