In a large nonstick skillet, sauté onion, garlic, carrot, and mushrooms over medium heat, stirring occasionally, until vegetables are wilted and pan liquids are absorbed, about 6 minutes.
Crumble in the ground sirloin and continue to cook, stirring occasionally, until beef is well browned, about 5 minutes. Drain off any excess fat.
Add oregano, thyme, cumin, and pepper. Sprinkle flour over the beef mixture and stir. Add beef broth, wine, and tomato paste. Stir until well combined.
Line a 2-quart (2 l) casserole with a piece of heavy-duty aluminum foil large enough to completely enclose the casserole. Lightly spray the foil inside the casserole with cooking spray.
Transfer meat mixture to the prepared casserole. Set aside while you prepare the topping.
Place the potatoes in a pot of boiling water to cover. Reduce heat and simmer until tender when pierced with a fork, 15 to 20 minutes. Drain.
Mash the potatoes, adding sufficient skim milk until the potatoes are fluffy. Stir in the salt (if using), minced jalapeño chiles, cilantro, and scallion. Spoon the potato mixture onto the top of the meat mixture, making swirls with the back of a spoon. Sprinkle with paprika and cheese.
Cover the casserole and freeze until solid. Lift the food out of the casserole (the foil will adhere), wrap securely in the foil, then slip the package into a self-sealing plastic freezer bag or overwrap in additional foil or freezer paper. Label with the casserole name and the date. Use within 3 months.
When ready to bake, unwrap the casserole and place back in the casserole dish to bake. Defrost completely. Then bake at 375°F (190°C) for 40 to 45 minutes, until golden and bubbly. (Since the casserole will be full, it may bubble over, so place a sheet of aluminum foil under the casserole to catch any drips.) Serve hot.