Place salmon on a broiler pan coated with cooking spray; sprinkle with salt and pepper. Broil 7 minutes. Remove from oven, and spread 1 1/2 teaspoons mustard over each fillet. Broil 3 minutes or until salmon flakes easily when tested with a fork.
Place a small nonstick skillet coated with cooking spray over medium-high heat. Add onion and garlic, and sauté 1 to 2 minutes or until tender. Transfer onion mixture to a small bowl; stir in yogurt, mustard, and dill. Spoon yogurt mixture over salmon.