Preheat oven to 425°F.
Melt butter in large saucepan over medium heat. Add leeks and onion; cook 3 to 4 minutes. Add flour and cook, stirring constantly, 3 to 4 minutes. Do not let flour brown. Gradually add broth and cook, stirring constantly, until thickened. Add heavy cream, TABASCO® Green Sauce and nutmeg; cook over low heat until thick, about 3 minutes. Remove from heat.
Layer chicken and ham in 1 1/2-quart casserole or souffle dish. Top with leek cream sauce.
Roll out pastry and place on casserole dish. Trim pastry, leaving 1-inch border. Mix egg and water in small bowl. Brush edge of casserole dish with egg wash; press overhanging dough onto dish. Crimp pastry decoratively and brush top with egg wash. Cut steam vent in center. Place casserole on baking sheet and bake 20 to 25 minutes or until crust is golden. Serve immediately.
Makes 6 servings.