Remove papery outer skin from garlic (do not peel or separate cloves). Cut top off of bulb. Brush with oil. Wrap in heavy-duty foil. Bake at 425° for 30-35 minutes or until softened. Cool for 10-15 minutes.
Reduce heat to 350°. In a small saucepan, melt butter. Squeeze softened garlic into pan. Stir in the onion, lime juice, mustard, salt and pepper. Cook and stir over medium heat until onion is tender.
Arrange the fillets in an ungreased 11x7-in. baking dish; top with onion mixture. Bake, uncovered, for 30-35 minutes or until fish flakes easily with a fork.