Melt butter in a skillet over medium heat; add onion. Cook and stir until translucent, about 5 minutes. Add wine; simmer until slightly reduced but still juicy, about 5 minutes. Pour mixture into the bottom of a baking pan. Place seasoned halibut on top.
Preheat oven to 350 degrees F (175 degrees C).
Combine sour cream, mayonnaise, garlic, seafood seasoning, white pepper, paprika, and dill in a bowl. Season with salt. Pour on top of halibut. Sprinkle green onions, panko, and capers on top.
Bake in the preheated oven until the fish flakes easily with a fork, about 15 minutes.