In a medium mixing bowl, add the mayonnaise, sour cream, and finely chopped onion. Pour in the lemon juice, followed by the prepared horseradish. Sprinkle in the fresh dill, garlic salt, and freshly ground pepper.

Using a whisk or a spoon, stir the mixture vigorously until all ingredients are well combined and the texture is smooth.

Cover the bowl tightly with plastic wrap or transfer the sauce to an airtight container. Refrigerate the dill sauce for at least 30 minutes.

After chilling, use the creamy dill sauce generously over your prepared salmon, whether baked, grilled, or pan-seared.