Thaw the fish, rinse and dry with paper towels. Wash the onions and garlic, peel them, finely chop the garlic, and chop the onion into rings. Wash the lemon and cut it in half, cutting one half into slices. Prepare a marinade bowl and cooking foil. Preheat an electric grill or light a charcoal grill.
Prepare the fish in the marinade. In a bowl, combine olive oil, balsamic vinegar, soy sauce, pepper and salt. Squeeze the juice from half a lemon. Add fish to the mixture. Brush tuna pieces with marinade on all sides. Leave the fish to marinate for 30 minutes.
Grill the steaks. Unfold a sheet of foil and place the fish on it. Sprinkle the steaks with thyme, dry garlic, and add lemon slices. Wrap steaks in foil and place on grill grate. Grill them for 3 minutes on each side.
Prepare the salad to serve. Mix beans, finely chopped garlic and red onion cut into rings, season with oil and vinegar, and add salt.
A bean and red onion salad will highlight the taste of grilled tuna and make the dish more sophisticated.