Preheat your oven to 375F and move the rack to the top third of the oven. Boil a salted pot of water and cook the ziti for 2-3 minutes less than package directions indicate. Drain and set aside once it's done.
Meanwhile, add the sausage meat and onion to another pot and cook until browned (about 7-10 minutes), stirring occasionally.
Add the garlic, Italian seasoning, and red pepper flakes to the pot and cook for about a minute.
Stir in the crushed tomatoes, cream, salt & pepper, and sugar. Let it heat through (about 5 minutes).
Drain the pasta, add it to the pot with the sauce, and toss.
Add half the pasta mixture to a 9x13 casserole dish (spread it out evenly to create your bottom layer).
Spoon the ricotta over the pasta for the cheesy middle layer (you don't need to be too neat... I just put dollops here and there).
Spoon the remaining pasta into the casserole dish to create an even top layer (I don't stir it).
Sprinkle the parmesan and mozzarella over top.
Bake, uncovered, for 20 minutes (or until hot & bubbly), and then broil (watch it closely) to brown the cheese.
Serve with some fresh basil and/or parsley if desired.