In a small bowl, combine the yogurt, mayonnaise, dill and mustard; cover and refrigerate. Sprinkle halibut with salt and pepper. Using long-handled tongs, moisten a paper towel with cooking oil and lightly coat the grill rack. Grill halibut, over medium heat or broil 4 in. from the heat for 4-6 minutes on each side or until fish flakes easily with a fork.