Preheat the oven to 375 degrees F. If you're not using an oven-safe skillet, set aside a 9x13 baking dish.
Bring a large pot of well-salted water to a rolling boil. Cook the pasta to al dente according to the package directions. Reserve 1 cup of pasta water. Drain.
Meanwhile, heat the olive oil in a large, preferably oven-safe skillet or casserole, over medium heat.
Add the sausage and cook, breaking it up with a wooden spoon, for 8 to 10 minutes or until cooked through and lightly browned. Remove to a plate leaving behind drippings.
Add the diced onion to the pan with the sausage dripping and cook for 3 to 4 minutes or until softened. Add the minced garlic and cook for another 30 seconds.
Add the tomato paste to the pan with the onions and garlic and cook, stirring often, for 1 minute.
Pour in the crushed tomatoes, brown sugar, Italian seasoning, salt, and pepper and mix to combine. Mix in the cooked sausage.
Add the drained pasta and 1/2 cup of the reserved pasta water. Toss until all of the pasta is coated in sauce. Add more pasta water if necessary to create a velvety but not runny sauce.
If necessary, transfer the pasta to a 9x13 baking dish.
Top with shredded mozzarella and parmesan cheese.
Bake uncovered for 30 minutes or until the cheese has melted and is golden brown and the sauce around the edges is bubbling.
Allow the pasta to rest for 10 to 15 minutes before serving. Sprinkle with chopped parsley and serve warm.
Notes
If you forgot to reserve the pasta water, use 1/4 to 1/2 cup of plain water.
Store in an airtight container in the fridge for up to 4 days.
Reheat in 30-second bursts in the microwave or in a 350-degree oven for about 10 minutes or until warmed through.