Rinse fish and pat dry. Season lightly with salt and pepper. Fill the cavity of each fish with 2 pieces of lemon, herbs, onion, tomato, garlic and butter. Lightly brush the outside of the fish with olive oil. Place in fish basket.
Place the fish in the fish basket on the grids, and reduce heat to MEDIUM-LOW. Cook, turning once, until fish turns opaque and starts to flake, approximately 8 minutes per side, depending on the thickness of the fish.