If using fresh vegetables like carrots and celery, steam them or cook them in the frying pan with the browned beef until tender. Thaw frozen vegetables before adding them. You can use any combination of vegetables you prefer. Substitute canned mushroom soup with other "cream of" soups, gravy, or even tomato soup. Any size casserole dish works for this recipe, though a deeper dish may need a bit more cooking time. You can assemble this casserole ahead of time and refrigerate it; if baking it straight from the fridge, allow for additional baking time.