1. Place sliced onions and sugar into a large, heavy-bottomed pot, over low heat. Stir until sugar is melted, then add all the Chablis. The whole bottle. Add peppercorns and bay leaves. Cover; bring to a boil. Add lemon juice. Gently place the fish into the pot, skin side up. Bring to a gentle simmer; cook for 30-35 minutes, covered. Allow to cool.
2. After fish has cooled, transfer to a container (I like to use glass Pyrex) skin side up (to help the fish stay flavorful and moist). Strain the pan liquids through a sieve; pour the strained liquid over the fish. Reserve some bay leaves to garnish your plates. Discard the onion. Cover; refrigerate until ready to serve.
Notes
Note: Chablis is an inexpensive wine that gives this fish a great, foolproof taste. It can be found in all wine stores. Your fish will always come out consistently good. Because there is no water, the measurements are always the same, no matter the size of the pot. I do not recommend substituting with any other white wine.