Heat the olive oil in a large skillet over medium-high heat. Add the Italian sausages and brown them on all sides, about 8-10 minutes. They don't need to be cooked through at this stage. Remove sausages from the skillet and set aside on a plate, leaving any rendered fat in the pan.

Reduce heat to medium. Add the sliced yellow onion to the skillet and cook, stirring occasionally, until softened and lightly golden, about 5-7 minutes. Add the minced garlic and cook for another 1 minute until fragrant, being careful not to burn it.

Stir in the sliced bell peppers (red, yellow, orange, and green). Cook for 5-7 minutes, stirring occasionally, until they begin to soften slightly but still have a bit of crispness.

Pour in the crushed tomatoes and chicken broth. Stir in the dried oregano, dried basil, red pepper flakes (if using), sugar (if using), salt, and black pepper. Bring the sauce to a gentle simmer, scraping up any browned bits from the bottom of the pan.

Return the browned sausages to the skillet, nestling them into the sauce and vegetables. Reduce the heat to low, cover the skillet, and let it simmer for 25-30 minutes, or until the sausages are cooked through and the peppers are tender. Stir occasionally to prevent sticking and ensure even cooking.

Taste the sauce and adjust seasoning (salt, pepper, sugar) if needed. Garnish with fresh chopped parsley. Serve hot, directly from the skillet, with crusty bread or on sub rolls for a classic sandwich.