Preheat oven to 375°F. Cut the tops off the peppers and scoop out the seeds and membranes. Use a small paring knife to carve triangle eyes and a jagged mouth into each pepper like a jack-o'-lantern.
Brown the ground beef with the onion and garlic in a large skillet over medium-high heat, 6–8 minutes, breaking it up as it cooks. Drain off excess fat.
Stir in the diced tomatoes, cooked rice, Italian seasoning, salt, and black pepper. Simmer 5 minutes.
Spoon the filling into the carved peppers and set them upright in a baking dish.
Sprinkle the mozzarella over the top of each pepper. Bake 25–30 minutes until the peppers are tender and the cheese is golden and bubbly.