Rinse fish and pat dry. Combine yellow onions, thyme, garlic, salt and pepper in a large, deep skillet. Add the skate wings and cover with cold water. Cover and bring to a boil over high heat. As soon as water boils, remove pan from heat. Remove 1 cup of the liquid into a small nonreactive saucepan and set aside. Cover the skillet and set aside until the fish is cooked through, about 10 minutes.
Meanwhile steam the cabbage until cooked, about 10 minutes. Bring the reserved cup of liquid to a boil over high heat. Boil until reduced by half, about 5 minutes. Add the vinegar and boil for another minute. Swirl in the butter a few pieces at a time, and keep the sauce warm over very low heat. To serve, drain the skate on a towel. Remove the skin from both sides. Place 2 wedges of cabbage on each of 4 warmed dinner plates. Place skate on top of cabbage. Spoon sauce over the fish, sprinkle with leeks and serve immediately.