Heat the oil in a large skillet over medium heat. Add the onions and saute until they start to soften (~5 minutes). Stir in the garlic and mix until combined.

Add the spicy sausage and break it up with a wooden spatula as it browns. Saute until cooked through (~5 minutes).

Stir in the oregano, crushed red pepper, 1/2 tsp salt, and 1 tsp freshly ground black pepper. Then, add the diced tomatoes, tomato sauce, and stir until thoroughly combined.

Simmer uncovered for 15-20 minutes while the pasta is cooking. Stir occasionally to make sure nothing is sticking to the bottom of the pan. Adjust heat level accordingly. Sauce is ready when the liquid from the diced tomatoes has evaporated and sauce has thickened.

Cook the pasta to al dente according to package instructions. Make sure to salt the pasta water! When draining, keep 1/4 cup of the pasta water in case you need to thin out the sauce later.

Right before adding in the cooked pasta, stir in the butter until it has completely melted. At this point, salt and pepper the sauce to taste. Once it is seasoned to your preference, add in the pasta. Gently mix until combined and let the pasta simmer in the sauce for 1-2 minutes. If the sauce is sticking to the pan, use the reserved pasta water to thin it out. When the pasta is ready, the sauce should not be too liquidy!

Top with basil and freshly grated Parmesan cheese.