Bring a large pot of salted water to a rolling boil, then add the rigatoni. Cook for about 9-10 minutes or until al dente. Reserve ½ cup of pasta cooking water before draining.

In a large skillet, heat olive oil over medium heat. Add diced onion and sauté for 5-6 minutes until translucent. Stir in minced garlic and cook for an additional 30 seconds.

Incorporate tomato paste and crushed tomatoes into the onion and garlic mixture. Add Italian seasoning, black pepper, and red pepper flakes (if using). Simmer for about 10 minutes.

Remove the skillet from heat and transfer marinara sauce into a blender. Add cottage cheese and grated Parmesan cheese, then blend until completely smooth.

Return blended sauce to the skillet over low heat, add drained rigatoni, and toss gently to coat. Adjust sauce consistency with reserved pasta water as needed.

Taste and adjust seasoning if necessary. Serve hot, garnished with chopped parsley and additional Parmesan if desired.