Aside from the cheese, isn't everything in a lasagna already cooked? If so, why do recipes still call for 30-40 minutes of baking?
And how does the baking affect it? (e.g. makes it chewier?)
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Besides cooking some of the components (if you roll your own pasta or use some dry pasta sheets that don't have to be pre-cooked and can be assembled directly as long as they're submerged in sauce or they'll burn) the point of baking is to enhance flavor by forming a crust on top and to melt any cheese that you put between the layers (e.g. grated parmesan, some put mozzarella or provolone cubes). But mostly crust and flavor.
Personal opinion: as Italian, I could not think of a crustless lasagna.
The time is meant to be a guide. Longer bake times are meant to cook the lasagna sheets if you haven't precooked them already. A second reason for longer baking is to reduce the sauce if the sauce hasn't been reduced properly. If you have cooked everything before hand and the sauce is perfect, you can bake it for just 20 mins to heat it thoroughly. Also, the longer bake lets the flavors mix so that the pasta absorbs the sauce.