Preheat oven to 350 degrees. Slice the onion into 1/4-inch slices. Slice the cabbage into 1/2-inch slices. Heat 2 tablespoons butter in a large frying pan. Add the onions and cook over medium high heat until they just begin to brown. Add the garlic and cabbage, season with salt and pepper and reduce the heat to low. Continue cooking about 40 minutes, or until the vegetables are tender and golden. Add the thyme leaves and set aside. Using a Mandoline or a very sharp knife, cut the potatoes into the paper thin slices. Season with salt and pepper and toss with olive oil. Cut 4 pieces of parchment paper into double layered heart shapes with a fold down the center, about 8 inches long. Open up the parchment and lay out the potatoes on one side in a single layer. Set a spoonful of cabbage mixture on top, sprinkle with sherry vinegar and then set a piece of halibut on top of the cabbage. Season the fish with salt and pepper and then sprinkle with chopped mint. Toss the carrots with the shallots, salt, pepper and olive oil and distribute among the packages on top of the fish. Toss in the capers. Close the packages by making tiny folds along the edges of the parchment paper hearts. Bake on sheet pans 15 minutes or until fish is cooked through. Cut the butter into 1/4-inch circles. When the fish are done, make a cross in the top of the paper and open it up. Slip a slice of anchovy butter in on top of the fish, garnish with a lemon wedge and a sprig of parsley and serve