In a large skillet, brown the ground turkey or beef over medium heat, breaking it up with a spoon. Drain excess fat if necessary.
Add the diced onion, minced garlic, bell pepper, and sliced mushrooms to the skillet. Sauté for about 5-7 minutes until softened.
Stir in the drained diced tomatoes, pizza sauce, Italian seasoning, oregano, and red pepper flakes. Cook for an additional 3-5 minutes until heated through. Season with salt and pepper to taste.
In a 9x13-inch baking dish, spread half of the meat and vegetable mixture in an even layer.
Sprinkle half of the mozzarella cheese on top of the meat mixture.
Repeat the layers with the remaining meat mixture and top with the remaining mozzarella cheese.
Scatter the sliced black olives on top, then cover the dish with aluminum foil.
Bake in the preheated oven for 25 minutes.
After 25 minutes, remove the foil and bake for an additional 10-15 minutes until the cheese is bubbly and golden.
Remove from the oven and let it cool slightly. Garnish with fresh basil leaves if desired before serving.