Preheat oven to 350 degrees.
Bring a large pot of water, with 1 Tablespoon of salt, to a boil. Cook pasta according to package. Drain well, then return pasta to original pot, add 1 jar of prepared spaghetti sauce and combine. Set aside.
In a large skillet, heat 2 tablespoons of olive oil over medium heat until it shimmers. Add onion and garlic and cook until they begin to soften, about 5 minutes. Add sausage to skillet and brown, crumbing sausage with a wooden spoon. Cook until there is no longer any pink in sausage. Drain well. Add 1 jar of spaghetti sauce to skillet. Set aside.
In a medium mixing bowl, combine ricotta cheese, cream cheese, sour cream, 1 cup mozzarella, and Italian seasoning. Set aside.
Place half the slice of butter in a baking dish. Spread half the spaghetti dish, then spread cheese mixture evenly over spaghetti. Top with the remaining butter slices. Spread remaining spaghetti over cheese mixture. Pour tomato meat sauce evenly over top Layer of spaghetti.
Top with remaining mozzarella and bake in preheated oven until casserole is heated through, about 35-45 minutes.