Prepare flavored butter of your choice.
Rinse fish, pat dry, and rub with oil.
Place on a lightly greased grill 4 to 6 inches above a solid bed of hot coals.
Cook, turning once or twice, until fish is just opaque or tuna is slightly pink but still moist in thickest part; cut to test 8 to 10 minutes.
Season fish to taste with salt and pepper; offer flavored butter to spoon over individual portions.