Preheat oven to 425F/215C.
Place ½ of hash brown in casserole and bake for 7 minutes. Remove from oven.
In a nonstick skillet heat 1 tsp oil and cook ground beef for 5 minutes, until no longer pink. Remove from pan and reserve. In same pan cook onions and mushrooms in 1 tsp oil, until tender and water has evaporated from mushrooms.
In a bowl, combine beef, mixed vegetables, onion mixture, soup, sour cream and milk. Place in casserole. Top 1/3 of casserole with spinach. Sprinkle cheese over casserole and top with remaining hash browns.
Bake for 30 to 35 minutes, until golden and bubbling.