In a glass dish, whisk together the lemon juice and olive oil along with some salt and pepper. Add the fish, cover and refrigerate for about 1 hour, turning once.
In a microwave safe measuring cup, microwave the parsley and butter together until the butter is melted; remove the fish from the marinade and pour the marinade into melted butter/parsley. Mix well.
Heat the BBQ or a grill pan to high heat (use a paper towel to oil the grids prior to starting); place the fish on the grids and cook, basting often with the marinade mixture; until just cooked through. (The middle of the fish should be slightly opaque, do not over-cook). Serve with lemon slices, if desired.
Per Serving: 332 Calories; 23g Fat (6g Sat, 13g Mono, 3g Poly); 30g Protein; 2g Carbohydrate; trace Dietary Fiber; 61mg Cholesterol; 347mg Sodium. Food Exchanges: 4 1/2 Lean Meat; 4 Fat.