I want to make a butternut lasagna which calls for sausage. Is there something I can substitute the sausage with to make it vegetarian that will still taste great??
You can substitute the sausage meat itself with any suitable veg that's normally used as the star of a dish, such as eggplants or firmer squashes, however if you want that sausage taste the principle flavoring spices of Italian sausage are caraway seed, sage and sometimes paprika. If you want to add an umami element (meaty taste) substitute all salt with soy sauce, and perhaps add a little extra soy (experiment) to get the level of taste you want without making the lasagna over watery.
One thing always worth trying when sausage or bacon is called for is smoked tofu. It usually comes very firm, which suits the purpose well. I would recommend shallow-frying it before adding it to a sauce/filling if using big pieces, or putting it in the oil a few minutes ahead of other aromatics if using brunoise/small dice-size pieces. In the second case, try adding brunoised/diced standard mushrooms and maybe some (fresh or reconstituted) shiitake too; also make sure these are well fried.
Depending on your locality, you can certainly substitute in vegetarian Italian sausage made with something like soy instead of meat.
This will give you a really similar taste and texture to the sausage and won't add a bunch of extra water, which would be an issue with some other vegetable options.
Other options for true "sausage" substitutes would be to follow a recipe to make your own, like this one, made primarily out of mushrooms and black-eyed peas or this one made from vital wheat gluten (though it can be difficult to find for some people).
As a note, I do not endorse any of these products or recipes as I have not tried them myself.
The answer to this is: anything, except meat. The rest is a matter of taste.
Let me give a few suggestions. If you want to stay close to the meaty idea, you could use garlic, rosemary, sage, and cumin (cumin goes really well with squash, countless recipes and cultures make this combination). I would put in some nuts as well, preferably pistacchios. Anything really, except peanuts.
Or you could go the other way and use spinach or kale. Or perhaps Cauliflower puree.