Add butter to a large skillet or dutch oven over medium high heat; add the onion, green pepper, and fennel. Cook for 5-7 minutes until tender. Add ground beef and cook until brown. Drain most of the fat and discard.
Add the remaining ingredients and simmer for 15 minutes, adding more salt and pepper to taste.
Toast buns. Spoon meat mixture over the rolls; optional to top with a slice of cheese.