Whisk sour cream, dill, onion, mustard, and lemon juice in a small bowl.
Preheat oven to 400 degrees F.
Lightly oil baking sheet with cooking spray.
Place salmon, skin-side down, on prepared sheet.
Sprinkle with garlic and pepper, then spread with the sauce.
Bake salmon until just opaque in the center, about 20 minutes.