Combine potato, carrot, green pepper, onion, 1 cup water, and 1/2 teaspoon salt in a small saucepan. Simmer, uncovered, 15 minutes, stirring occasionally. Stir in peas. Drain vegetables, reserving cooking liquid. Set vegetables and liquid aside.
Melt butter in a large saucepan over low heat; add 2 tablespoons flour, stirring until smooth. Cook 1 minute, stirring constantly. Add milk to reserved cooking liquid to equal 2 cups; gradually add to flour mixture. Cook over medium heat, stirring constantly, until thickened and bubbly. Stir in 1/2 teaspoon salt and pepper. Gently fold in tuna and reserved vegetables. Pour mixture into a lightly greased 1 1/2-quart casserole, and set aside.
Combine remaining flour and salt in a small mixing bowl. Cut in shortening with a pastry blender until mixture resembles coarse meal. Sprinkle with ice water, and stir with a fork just until all dry ingredients are moistened. Shape into a ball.
Roll pastry out on a lightly floured surface to fit top of casserole. Carefully place pastry over top of dish, crimping edges to seal. Make several slits in pastry to allow steam to escape. Bake at 450° for 20 minutes or until pastry is lightly browned. Remove from oven, and serve hot.