Heat 1 tablespoon oil over medium-high in a 12-inch skillet.
Cook sausage, breaking it up, until browned, about 5 minutes.
Transfer sausage to slow cooker.
Add onions, garlic, tomato paste, oregano, and red pepper flakes to skillet.
Cook over medium until onions soften and brown, 5-7 minutes.
Stir in wine, scraping up browned bits.
Simmer until thickened, about 5 minutes.
Transfer mixture to slow cooker.
Add crushed tomatoes, diced tomatoes, and tomato puree to slow cooker.
Cook 8-10 hours on low or 5-7 hours on high.
Skim excess fat from sauce.
Toss bell peppers with 1 tablespoon oil in a microwave-safe bowl.
Microwave, stirring occasionally, until tender, about 5 minutes.
Stir bell peppers into sauce.
Season with salt, pepper, and sugar to taste.
Stir in 2 tablespoons parsley for every 6 cups of sauce before serving.