Sausage casseroles I make slightly differently to something like chicken, for example. For a sausage casserole I make a smooth tomato sauce (onions, tomatoes, basil, pepper, mixed spice, balsamic vinegar, blitz it) and then put a few raw sausages in a dish, pour the sauce over the top and bake until the sausages go crispy on top and lovely and soft on the bottom.
OR, I guess you could make it as you would a chicken casserole. Heat up a frying pan, put onions in there with the sausages, and let the sausages cook a tiny bit, and then transfer it to the casserole dish, pour over tomatoes, season with pepper (no salt needed, the sausages are salty) and basil/majoram,add diced carrots and mushrooms, and then bake in the oven for up to an hour, depending on your oven.