In a large dutch oven or skillet over medium heat, cook the Italian sausage, breaking it into small crumbles, until browned and cooked through (about 6-8 minutes). Remove the sausage from the pan and set aside.
In the same skillet, add the diced onions and mushrooms. Sauté over medium heat for about 3-4 minutes, until the onions are translucent and the mushrooms are soft. Add the minced garlic and cook for another 1-2 minutes until fragrant.
Stir in the tomato paste and cook for 2-3 minutes to deepen its flavor.
Pour in the white wine (or chicken stock) and stir, scraping up any browned bits from the bottom of the skillet. Let the liquid simmer for 2-3 minutes to reduce slightly.
Add the chicken stock, cooked sausage, Italian seasoning, and crushed red pepper flakes (if using). Stir well and let the mixture simmer for about 5 minutes to meld the flavors together.
Reduce the heat to low, then stir in the heavy cream and grated Parmesan. Let the sauce simmer gently for another 2-3 minutes until it thickens. Taste and season with salt and pepper as needed.