In a bowl, combine flour and salt; cut in shortening until the mixture resembles coarse crumbs. Add mint. Add water, 1 tablespoon at a time, until mixture can be formed into a ball. On a lightly floured surface, roll dough to fit a 9-in. pie plate. Bake at 425° for 12 minutes; set aside.
In a large skillet, brown lamb in shortening; sprinkle with salt and pepper. Add 1 cup of water and mushrooms; cover and simmer for 45-60 minutes or until meat is tender.
Meanwhile, place peas, carrots and onions in a large saucepan; cover with water. Cover and cook until tender; drain. Spoon into the crust. Using a slotted spoon, remove lamb and mushrooms from skillet and place over vegetable. Combine flour and remaining water until smooth; stir into pan drippings. Cook and stir until thick and bubbly. Pour over lamb mixture. Serve with a spoon.