Preheat oven to 375 degrees. Rinse fish under cold running water, inside and out, being sure to remove all traces of gills and blood. Blot dry with paper towels, inside and out. Lightly season, inside and out, with salt and pepper.
Lightly oil the bottom of a large roasting pan. Cover the bottom of the pan with half of the sauce. Place fish over sauce. Tuck bay leaves, thyme, and onion into the cavity of the fish. Pour remaining sauce over fish.
Bake just until the flesh flakes when tested with the tip of a knife, 30 to 40 minutes. Use two large spatulas to transfer fish to a platter spooning sauce and cooking juices over it. Or remove fish from bone and serve on individual plates, with sauce spooned over it.