Heat olive oil in a large skillet over medium heat.
Add chopped onion and sauté until translucent.
Stir in minced garlic and cook for 1 minute.
Add sliced bell pepper and cook until softened.
Remove sausage from casings and add to the skillet.
Break sausage into smaller pieces and brown evenly.
Stir in diced tomatoes, oregano, and fennel seeds.
Season with salt and pepper; simmer for 15 minutes.
Serve over cooked pasta or in a sandwich.
Garnish with fresh parsley before serving.