In a large pot of boiling salted water, cook pasta according to package instructions; reserve 1/2 cup water and drain well.
Melt butter in a large cast iron skillet over medium high heat. Add onion to the skillet, and cook, stirring frequently, until translucent, about 2-3 minutes.
Stir in tomato paste, garlic, Italian seasoning and crushed red pepper flakes until the tomato paste begins to brown and caramelize, about 4-5 minutes.
Stir in wine, scraping any browned bits from the bottom of the skillet.
Stir in tomatoes and sugar. Bring to a boil; reduce heat and simmer, stirring occasionally, until reduced and thickened, about 8-10 minutes.
Stir in heavy cream and Parmesan until well combined and cheese has melted, about 1 minute.
Stir in pasta and reserved pasta water, about 1/4 cup at a time, until desired consistency is reached; season with salt and pepper, to taste.
Serve immediately, garnished with parsley, if desired.