Bring 6 cups water with wine, onion, bay leaves, peppercorns, thyme, and salt to a boil in a deep 12-inch skillet, covered, over moderate heat, then reduce heat and simmer 10 minutes.
Add halibut (liquid should barely cover fish; if not, add more water) and poach at a bare simmer, uncovered, until opaque and just cooked through, about 6 minutes.
Transfer fish to a plate with a slotted spatula. Peel off and discard skin, then transfer fish to a platter or individual plates with slotted spatula.