Unfold pastry on a lightly floured board. If necessary, roll out or cut pastry to fit snugly inside top of a shallow 3-quart casserole (9 by 13 in.). Transfer pastry to a 12- by 15-inch nonstick baking sheet. Cut through pastry in several places to make a decorative pattern. Brush lightly with milk and sprinkle evenly with cheese. Cover with plastic wrap and freeze.
Rinse zucchini, and trim and discard ends; cut into 3/4-inch chunks. Peel onion and cut into 3/4-inch chunks. Discard chipotle chili stems; if desired for less heat, also remove veins and seeds. Chop chilies.
Uncover pastry and bake in a 425° regular or convection oven until golden brown, 10 to 15 minutes.
Meanwhile, in a 5- to 6-quart pan over high heat, frequently stir zucchini, onion, and cumin in olive oil until vegetables are lightly browned, 7 to 9 minutes. Add tomatoes, including juice, and hominy; bring to a boil.
Discard any fat from pot roast and sauce. Scrape gravy from beef and reserve. Cut beef into 3/4-inch chunks.
Add beef and gravy to pan and mix; cover and bring to a boil. Reduce heat to low and simmer, stirring occasionally, until beef is hot, 7 to 9 minutes. Add chilies to taste. In a small bowl, mix cornstarch and 3 tablespoons water. Add to pan and stir until boiling.
Pour hot mixture into a warmed shallow casserole. Set hot pastry on filling. Use a large spoon to break through pastry and scoop out portions. Add salt and pepper to taste.