Start by bringing a large pot of salted water to a boil. Once boiling, add the fusilli bucati pasta and cook until al dente. Reserve 1 cup of cooking water, then drain the pasta and set aside.
In a skillet, heat the olive oil over medium heat. If using onion, add and sauté until soft and translucent, about 4-5 minutes. Stir in minced garlic and cook for an additional minute.
Add the tomato paste to the skillet, stirring into the sautéed onion and garlic. Cook for about 2-3 minutes, allowing the tomato paste to caramelize slightly.
Pour in the heavy cream and chicken stock, stirring until combined. Bring to a gentle simmer. Add red pepper flakes if desired. Simmer for about 5 minutes, thickening slightly. Taste and adjust seasonings.
Add the cooked pasta to the skillet and toss to coat in the sauce. If too thick, gradually add reserved pasta water until desired consistency is reached.
Stir in freshly grated Parmesan until melted and well incorporated. Serve immediately, garnished with additional Parmesan if desired.
