1. Oil the grill grates or a fish basket and prepare grill to medium-high heat.
2. Sprinkle fish with salt to taste.
3. In a bowl, whisk together the olive oil, mustard, lemon juice, and black pepper. Spread this mixture on fillets until covered well.
4. When grill is preheated, place fish fillets on grate (or in basket) over indirect heat, cover grill, and cook until fillets are opaque and flake easily, turning once during cooking time. This should take a total of about 10 to 12 minutes depending on the size or thickness of fillets.