Bring a large pot of salted water to a boil. Add rigatoni and cook until al dente, undercooking by 1 minute since it will finish in the sauce. Reserve ½ cup pasta water, then drain.

Heat olive oil in a large skillet over medium heat. Add onion and sauté 5–6 minutes until softened and golden. Stir in garlic and cook 30 seconds until fragrant.

Stir in tomato paste, crushed tomatoes, Italian seasoning, red pepper flakes (if using), salt, and black pepper. Simmer for 10 minutes, stirring occasionally, until the marinara thickens slightly.

Transfer the hot marinara to a blender (or use an immersion blender). Add cottage cheese and Parmesan. Blend until completely smooth and creamy, creating a silky high-protein pasta sauce. Blend while the sauce is still hot for the creamiest texture.

Return the creamy cottage cheese marinara sauce to the skillet. Add cooked rigatoni and toss until every piece is evenly coated. Adjust consistency with reserved pasta water as needed.

Taste and adjust seasoning. Garnish with fresh parsley and extra Parmesan. Serve immediately as a hearty, healthy pasta dish.