Preheat an outdoor grill to medium heat and oil the grates with a brush or towel.
In a small saucepan, melt the butter and add the smashed garlic.
Make 3 deep cuts on each side of the fish. Season fillets generously with salt and pepper. Place flesh side-down and cook for 2 to 3 minutes. Flip fish over and cook for an additional 2 to 3 minutes. Remove from grill and brush with some of the garlic butter (about 2 tablespoons).
Squeeze 1/2 of a lime over each fillet and serve immediately.