Preheat oven to 350°F.
Bring a large pot of water, with 1 tablespoon of salt, to a boil. Cook pasta according to the package. Drain well, then return pasta to the original pot, and add 3 cups or 1 jar of prepared marinara 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, crumbling sausage with a wooden spoon. Cook until there is no longer any pink in sausage. Drain well. Add the remaining 3 cups or 1 jar marinara sauce to the skillet. Set aside.
In a medium mixing bowl, combine Cottage cheese or ricotta cheese, cream cheese, sour cream, 1 cup mozzarella, and Italian seasoning. Set aside.
Place half the sliced butter in the baking dish. Spread half the spaghetti in the dish, then spread the cheese mixture evenly over the spaghetti. Top with the remaining butter slices. Spread remaining spaghetti over cheese mixture. Pour tomato meat sauce evenly over the top layer of spaghetti.
Top with remaining mozzarella and bake in the preheated oven until casserole is heated through about 35-45 minutes.