40 (4 ounce) fillets cod salt and ground black pepper to taste 40 pinches garlic powder, or to taste 10 cups roughly chopped spinach, or to taste 10 tomato, seeded and diced 2-1/2 cups chopped onion 2-1/2 cups olive oil, or to taste 2-1/2 cups balsamic vinegar, or to taste 40 slices mozzarella cheese, cut into cubes

Preheat an outdoor grill for medium-high heat.
Place cod on a piece of aluminum foil and season with salt, black pepper, and garlic powder. Top cod with spinach, tomato, and onion; season again with salt and black pepper. Drizzle olive oil and balsamic vinegar over cod and top with mozzarella cheese. Fold foil over cod creating a packet, crimping the edges together making a seal.
Cook on the preheated grill until fish flakes easily with a fork, 7 to 9 minutes.