Start by boiling a big pot of water for the pasta. Once bubbling, toss in the short pasta you picked. Think al dente for that perfect bite because it will soften more in the oven. Do not forget to stir so it does not stick together.
In a skillet, crank up the heat and add the Italian sausage. It should start sizzling in just a minute, releasing that savory aroma. Keep breaking it apart and stir until it is browned. Be cautious; it can overcook and dry out if you leave it too long.
Toss in the diced onion and minced garlic into the same skillet after the sausage is cooked. Let it sauté until everything is fragrant and the onion is soft. Take care not to burn the garlic, or it will get bitter.
Grab a big bowl and add the ricotta, Parmesan, and two cups of mozzarella. Toss in the egg, Italian seasoning, salt, pepper, and red pepper flakes. Stir until it looks creamy and inviting; do not skip on that mixing or it will not be as delicious!
Time to stir it all together! Add the drained pasta and sausage mixture into the cheese filling. Then, fold in the marinara sauce gently until it looks like a perfect blend. If over-mixed, the pasta may break and lose its charm.
Grab a baking dish and layer half of the pasta combo at the bottom, followed by the remaining mozzarella cheese. Then add the final layer of pasta on top. You will want to save some fresh basil for after baking, so hold onto it for now.
Cover the baking dish with foil and pop it into the oven. You want it bubbling and cheesy, which should take about half an hour. Make sure the foil is not touching the cheese or it might stick as it melts.
Once out of the oven, let the dish rest for a bit before serving. It will smell amazing, but allowing it to sit helps it firm up a bit. If you cut into it too soon, you might get a mess instead of neat portions.