Baked Salmon Dijon

1 C fat free sour cream
2 tsp dried dill
3 Tbsp scallions, finely chopped
2 Tbsp Dijon mustard
2 Tbsp lemon juice
1 1/2 lb salmon fillet with skin, cut in center
1/2 tsp garlic powder
1/2 tsp black pepper
as needed fat free cooking spray