In a blender or food processor, combine the mayonnaise, sour cream, chopped basil, chopped scallion, parsley, and 1/8 teaspoon of garlic powder. Blend until smooth.
If desired, thin the mixture with some cream or milk. Taste and add salt and pepper, as needed. Add extra garlic powder, if desired.
Put the mayonnaise mixture in a bowl or jar; cover and refrigerate until serving time.
Preheat the oven to 425 F.
Line a rimmed baking sheet with foil and spray it with nonstick cooking spray. Place the salmon fillets on it, skin-side down.
Brush the salmon fillets with lemon juice and sprinkle them lightly with salt and pepper.
Bake in the preheated oven for about 12 to 15 minutes, or until the fish is cooked through but still somewhat moist. If the fillets are quite thick, check with an instant-read thermometer. The minimum safe temperature is 145 F and they shouldn’t be much above that.
Serve the salmon with a drizzling of the basil and garlic mayonnaise and finish with some fresh chopped parsley.