Brown beef and onion, drain, add garlic for 30 seconds, then tomatoes, sauce, oregano, salt and pepper and simmer 8 minutes. Cool for 15 minutes.
Fold in cooled rice, frozen peas and 2 cups mozzarella. Spread in a freezer-safe 9-by-13-inch foil pan and top with remaining cheese.
Cool completely, wrap in plastic and two layers of foil, label and freeze up to 3 months. Remove plastic before baking.
Bake from frozen, covered, at 375°F for 50 minutes.
Uncover and bake 15 to 20 minutes more until bubbling and 165°F in the center. Rest 15 minutes.