Brown the Italian sausage. Drain any excess grease. (I move the sausage to one side, tilt the pan to drain oil and use paper towels to absorb it out of the pan.)
Add the onion and garlic and sauté over medium heat until translucent
Add tomatoes and red pepper flakes, simmering over low heat until tomatoes are softened and juice evaporated (about 30 minutes). Add heavy cream and continue to simmer while cooking pasta.
Meanwhile, prepare pasta al dente in salted water.
Prior to draining pasta, add 2-3 ladles of pasta water to sauce. Simmer over medium heat to desired consistency. More water will “loosen” the sauce. More simmering will “tighten” the sauce.