Cut tuna into portion-sized pieces (the red color spook will disappear when cooking). Place thinly sliced onion slices in a Pyrex dish. Place tuna pieces (1 1/2 to 2" thick) on top of onion slices. Cover with a bottle of Italian salad dressing; and marinate on each side for from 10 to 15 minutes. The charcoal fire is only ready when it is of uniform white. Grease grill with a stick of butter, then place tuna on grill without letting the pieces touch. Brush on marinade throughout cooking. Grill until well seared on one side then turn, baste and cook until meat is opaque throughout. Place pieces back in marinade pan until serving. It will look and taste more like a filet Mignon than fish. Any left overs make a delicious cold salad.