Combine onion and next 6 ingredients in a large bowl.
Cut four large pieces of parchment paper long enough to cover length of fish, and fold each in half lengthwise. Spoon onion mixture on bottom halves of parchment. Place one fish against the crease of each parchment piece, drizzle with white wine, and sprinkle with salt and pepper.
Fold the parchment at the bottom corner over itself. Continue folding over the previous fold to create a pleat and allow for the paper to be sealed tight. When you have formed a half circle and reached the opposite end of the crease, tuck the paper under the last fold. Place on a large baking sheet.
Bake at 350° for 20 to 25 minutes or until the fish is cooked through. Carefully tear paper open, allowing steam to escape.
Tip: If fish is too large to allow parchment paper to seal tight, place on aluminum foil and wrap foil up and around parchment edge, pinching to seal.