Melt 3 tablespoons unsalted butter in a Dutch oven or large pot over medium heat. Add 1 small diced yellow onion and cook, stirring occasionally, until softened, 3 to 5 minutes. Stir in 3 minced garlic cloves, 1 ½ teaspoons kosher salt, ¼ teaspoon black pepper, and ¼ teaspoon red pepper flakes if using. Cook until fragrant, about 30 seconds.

Stir in 1 (28-ounce) can crushed tomatoes and bring to a simmer. Reduce the heat to maintain a gentle simmer and cook, stirring occasionally, until the flavors meld, about 20 minutes. Meanwhile, cook the pasta.

Bring a large pot of salted water to a boil. Add 1 pound dried rigatoni pasta and cook 1 minute less than the package instructions for al dente. Reserve ½ cup of the pasta water. Drain the rigatoni.

Return the sauce to medium heat. Stir in ½ cup heavy cream and the reserved pasta water and bring to a simmer. Gently stir in the pasta until well-coated. Garnish with Parmesan cheese and chopped parsley leaves if desired.