Cook pasta in boiling, salted water per directions. Reserve 1/2 cup of pasta water before draining. (You'll cook the pasta as you are making the sauce.)
Heat a little olive oil over medium high heat (use more oil if your sausage is lean.)
Brown the sausage, stirring to break it up, about 4-5 minutes.
Add the onions, garlic and bell peppers and sprinkle with salt and pepper. Sauté for about 2-3 minutes, until the peppers are crisp tender.
Add the cooked pasta and a little of the pasta water if you want the pasta more moist. Taste and adjust for salt and pepper.
Serve with a fresh grating of Parmesan cheese.
I like to time it so that the pasta is ready right when the sauce is ready, then I just scoop the pasta out of the pot straight into the pan. This allows water to cling to the pasta and create a bit of a sauce.