Cook the pasta according to package instructions until al dente.
In a large skillet, heat olive oil over medium heat and sauté the diced onion until translucent.
Add minced garlic and sliced smoked sausage; cook until the sausage is browned.
Stir in the diced tomatoes and Italian seasoning; simmer for 5-7 minutes.
Add the cooked pasta to the skillet, tossing to combine with the sauce.
Fold in the cubed mozzarella and let it melt slightly.
Adjust the sauce with reserved pasta water if necessary; season with salt and pepper to taste.
Finish with fresh basil if desired, tossing gently before serving.