Prick tuna steaks all over with a fork and place in shallow glass dish. Whisk 1/2 cup vegetable oil, 1/3 cup soy sauce, 1/4 cup fresh lemon juice, 2 tsp Dijon mustard, 1 tsp grated lemon peel, and 1 clove garlic, crushed, together in a bowl; pour over the tuna steaks. Cover dish with plastic wrap and refrigerate 1 to 3 hours. Preheat grill for medium heat and lightly oil the grate. Remove tuna from the marinade. Pour the marinade into a small saucepan and bring to a boil and simmer for 10 minutes. Cook tuna on preheated grill, basting with boiled marinade, until cooked through, 5 to 6 minutes per side. Serve with lemon wedges.