Bring a medium pot of salted water to a boil. Wash and dry produce.
Heat a drizzle of olive oil in a large pan over medium-high heat. Add bell pepper and cook until lightly browned and slightly softened, 3-4 minutes.
Add sausage and scallion whites; cook, breaking up meat into pieces, until browned and cooked through, 4-6 minutes. Season with salt and pepper.
Once water is boiling, add rigatoni. Cook, stirring occasionally, until al dente, 9-11 minutes. Reserve ½ cup pasta cooking water, then drain.
Add cream sauce base to pan with sausage. Stir in 1 TBSP butter (2 TBSP for 4 servings) until melted. Season with salt and pepper.
Add drained rigatoni to pan with sauce; toss to coat. If needed, stir in reserved pasta cooking water a splash at a time until pasta is coated in a creamy sauce.
Divide pasta between bowls and top with Parmesan. Garnish with scallion greens and serve.