In A Small Bowl, Combine The Yogurt, Mayonnaise, Dill And Mustard; Cover And Refrigerate. Sprinkle Halibut With Salt And Pepper. Coat Grill Rack With Nonstick Cooking Spray Before Starting The Grill. Grill Halibut, Covered, Over Medium Heat For 4-6 Minutes On Each Side Or Until Fish Flakes Easily With A Fork. Serve Sauce With Halibut.