Sprinkle lamb with salt and pepper. Melt shortening in a large skillet over medium heat. Add lamb, and cook until browned, stirring frequently. Add 1 cup water and mushrooms; cover and simmer 30 minutes.
Combine remaining 3 tablespoons water and flour, stirring until smooth. Pour flour mixture into lamb mixture; cook, stirring frequently, until thickened and bubbly.
Cook carrots and onions in a small amount of boiling salted water until crisp-tender; drain well. Cook peas according to package directions; drain well.
Line cooked carrots upright around edge of pastry shell. Fill center of pastry with lamb mixture. Place peas and onions alternately around lamb mixture. Drizzle melted butter over vegetables. Serve immediately.