Cook the spaghetti according to the package instructions until al dente.
While the pasta cooks, heat olive oil in a large skillet over medium heat.
Remove the sausage from its casing and add it to the skillet, breaking it up into pieces as it cooks.
Once the sausage is browned, add the sliced onion and bell peppers to the skillet.
Stir in the minced garlic and red pepper flakes; cook for an additional minute until fragrant.
Pour in the diced tomatoes, and season with salt and pepper. Allow the mixture to simmer for about 10 minutes.
Once the pasta is cooked, drain and add it directly to the skillet with the sausage and pepper sauce.
If the sauce is too thick, add reserved pasta water a little at a time until desired consistency is achieved.
Serve hot, garnished with fresh basil leaves.