Heat olive oil in a large skillet over medium-high heat.
Brown the sausage on all sides until deeply golden. Transfer to a board.
Slice the browned sausages into thick pieces on a bias.
Add onions and peppers to the skillet with a pinch of salt. Cook until softened and lightly caramelized.
Stir in garlic and red pepper flakes. Cook 30 seconds until fragrant.
Pour in chicken broth and scrape up browned bits with a spoon.
Add crushed tomatoes, Italian seasoning, black pepper, and remaining salt. Return sausage to the pan.
Simmer gently until the sauce thickens and sausage is cooked through, about 10 minutes.
Finish with balsamic vinegar and parsley. Taste and adjust seasoning, then serve hot.