How to Make It
In a large bowl, using a pastry cutter or 2 knives, cut the butter into the flour until it resembles small peas. Add 6 tablespoons of the ice water and stir just until the dough can be gathered into a ball. Add more water, 1 tablespoon at a time, if the dough is too dry. Divide the dough into 12 equal portions and pat into 1/2-inch-thick disks. Wrap individually in plastic and refrigerate for at least 1 hour or overnight.
Working with 1 disk at a time (keep the rest refrigerated), roll out the dough on a floured surface to a 6-inch round and fit into a 1/2-cup ramekin or a 4-by-1-inch tart pan with a removable bottom. Trim the overhang. Refrigerate the pastry shells until well chilled.
Preheat the oven to 400°. In a large saucepan, melt the butter over moderate heat. Add the celery, leeks, onion and garlic and cook, stirring, until tender, about 10 minutes. Drain the oysters and reserve 1 cup of their liquor. Add the thyme and reserved oyster liquor to the pan and cook for 10 minutes. Add the oysters, bread crumbs and scallions and cook until the oysters are just cooked through, 3 to 5 minutes. Season with salt and pepper.
Spoon the filling evenly into the pastry shells and sprinkle each with 1 teaspoon of the cheese. Place the ramekins directly on a rack in the lower third of the oven and bake for about 25 minutes, or until the tops of the patties are golden and bubbling and the crusts are golden. To make sure the patties brown evenly, during baking move the front ramekins to the back of the oven and vice versa. Cool the patties slightly before serving or serve at room temperature. Pass the hot sauce separately.