Whisk sour cream, dill, onion and mustard in small bowl to blend. Season sauce to taste with salt and pepper. Let stand at room temperature 1 hour.
Preheat oven to 400°F. Lightly oil baking sheet. Place salmon, skin side down, on prepared sheet. Sprinkle with garlic, salt and pepper; spread with 1/3 cup sauce. Bake salmon until just opaque in center, about 20 minutes. Serve with remaining sauce.