Place haddock fillets on an 18x18-inch piece of aluminum foil; top with butter. Sprinkle onion, garlic, dill, and pepper over butter and haddock. Squeeze juice from 1/2 lemon evenly on top.
Lift up bottom and top ends of the aluminum foil towards the center; fold together to 1 inch above the haddock. Flatten short ends of the aluminum foil; fold over to within 1 inch of the sides of the haddock. Place foil package on a baking sheet.
Bake in the preheated oven until haddock flakes easily with a fish, about 45 minutes. Let sit, about 5 minutes. Open ends of the packet carefully; squeeze juice from the remaining 1/2 lemon on top.