Cook beef mince and sausage in a pan over medium-low heat until browned, stirring often to break up any clumps, 5 to 10 minutes. Mix in 3/4 of the tomatoes, onion, butter, garlic, oregano, fennel seed and seasoned pepper. Simmer over low heat until meat mixture comes together, about 20 minutes.
Preheat the oven to 180 C / Gas 4.
Fill peppers with meat mixture, alternating with layers of feta cheese and Parmesan cheese. Place stuffed peppers side-by-side in the prepared dish; add remaining tomatoes around and under the peppers so they cook up and into the peppers. Sprinkle with mozzarella cheese.
Bake in the preheated oven until browned and bubbly, about 30 minutes.