Preheat your oven to 375F and move the rack to the top third of the oven.
Boil a large, salted pot of water for the pasta. Cook it 2 minutes less than package directions indicate. Once it's done, drain it and add it back to the pot it was cooked in and set aside.
Meanwhile, add the sausage meat, along with the onion, to a skillet over medium-high heat. Cook it for about 10 minutes, breaking it up with your spoon as you go along and stirring occasionally, until it's cooked.
To the skillet, stir in the garlic and red pepper flakes, then continue cooking it for another minute or so.
To the skillet, stir in the marinara sauce and warm it through for a few minutes.
To the pot with the pasta, add in about half of the skillet sausage/sauce mixture and give it a stir. Transfer it to a 9x13 baking/casserole dish and spread it in an even layer.
Add the ricotta to the baking dish (you don't need to be too neat... I just use a spoon to put dollops here and there) and then sprinkle the parmesan evenly over top.
Spoon the rest of the sausage/sauce mixture evenly on top of the parmesan/ricotta layer.
Sprinkle on the mozza/cheddar for the final cheesy topping.
Bake, uncovered, for 20 minutes, and then broil for a few minutes to brown the cheese (watch it carefully). Let sit for about 5 minutes prior to serving, then enjoy!