Wash the bell peppers under cool running water. Remove stems and seeds, then slice into thick strips. Peel and slice yellow onions into thick strips. Mince garlic cloves finely.

In a large skillet, heat 2 tablespoons of olive oil over medium-high heat. Sear Italian sausage links for 2-3 minutes on each side until lightly golden. Transfer to a plate.

Layer sliced yellow onions at the bottom of the slow cooker, add sliced bell peppers, then sprinkle minced garlic evenly on top.

Arrange browned sausage links over the veggies. In a bowl, combine crushed tomatoes, diced tomatoes, tomato paste, basil, oregano, Italian seasoning, garlic powder, onion powder, smoked paprika, salt, and black pepper.

Stir in balsamic vinegar, and if desired, crushed red pepper flakes and sugar. Mix well to create marinara sauce.

Pour sauce over sausages ensuring they are mostly covered. Cook on LOW for 6 hours until sausages are tender.

After 6 hours, check for doneness of sausages. Stir vegetables into the thick marinara sauce.

For thicker sauce, simmer uncovered on HIGH for an additional 10 minutes. Adjust salt and pepper to taste.

Sprinkle fresh basil and parsley over the dish. Allow to rest for 5 minutes before serving.

Serve with freshly grated Parmesan cheese if desired. Enjoy warm!