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 and crimping the edges together to seal.

Cook on the preheated grill until fish flakes easily with a fork, 7 to 9 minutes.