Light a grill. In a small bowl, mix the mustard, horseradish and honey. Rub the salmon with oil and season with salt and pepper. Grill the salmon over moderate heat, skinned side down, until lightly browned, about 3 minutes. Turn and grill for 3 minutes longer, until the salmon is almost cooked through. Turn the salmon again and spread each fillet with 1 tablespoon of the horseradish glaze. Turn and grill until glazed, about 30 seconds. Serve the remaining glaze on the side.