Preheat oven to 180℃.
Heat a medium sized large skillet over medium high heat, add Italian sausage and brown. Remove sausage from pan and set aside.
Add onions and garlic to the pan used for the sausage and saute for 2 minutes. Add marinara sauce, sausage and asiago cheese, stir to combine reduce heat to low, cover and simmer.
While sauce is cooking, bring a medium sized sauce pan of salted water to a boil, add pasta and cook for 8 minutes.
Add cooked pasta to sauce and stir to combine and remove from heat.
Spread half of the pasta and sauce in the bottom of the casserole dish, sprinkle half of the.mozzarella cheese over the pasta. Spread the remaining pasta over the cheese and top with remaining mozzarella cheese.
Place casserole dish in the center rack of the oven and bake for 15 minutes. Move casserole to the top rack of the oven and increase temperature to 250℃, bake for 5 minutes.