I agree with JAZ. Make a dressing for the salad with the same ingredients as the marinade for the tuna, but leave out the garlic. I prefer to use something like sirloin or strip steak for grilling as it has more flavor. If you choose to stick with the filet, I think a compound butter is the way to go. Keep it simple, the cheese sauce is going to over power the flavors, and you already have cheese in the potatoes.