In a small bowl, combine the watercress and crabmeat and toss well. Set aside. Preheat the oven to 375 degrees and arrange the rack in the middle of the oven. On a generously floured surface, roll pastry from the center out, lifting the pastry, turning slightly, and occasionally flipping to prevent sticking. Roll pastry to 1/8-inch thickness. Lightly butter and flour a 8 by 12-inch tart pan with removable bottom, and line with pastry. Prick the bottom all over with a fork. Line the pastry with parchment or aluminum foil larger than the pan and fill the pan with weights, rice, or beans. Bake about 20 minutes, until the edges begin to color. Remove the paper and weights, and brush the mustard over the bottom of the pastry. Lower the oven temperature to 325 degrees F. In a bowl, beat eggs and yolks lightly with milk and cream, add sherry, nutmeg, salt, and pepper. Spoon crab mixture into tart shell, spreading evenly. Pour custard over crab to within 1/4-inch of the top of the crust. Bake 25 to 30 minutes, or until custard puffs. Allow to cool 10 minutes before serving. Remove tart shell and serve warm or room temperature.