Preheat oven to 350º
Heat a skillet over medium-high heat
Add ground beef and onion, and stir-fry
When beef is almost completely browned, add mushrooms, and continue to cook
When beef is browned, Stir in garlic
Add cream of mushroom soup, milk, salt, and pepper, and half of the fresh parsley, and stir until fully combined
Add a layer of potatoes to the bottom of a 9x13 inch casserole dish
Ladle ⅓ of the ground beef mixture over potatoes
Sprinkle with 1 cup of cheese
Repeat twice (potatoes, meat mixture, cheese)
Cover with foil, and bake for about 55 minutes, until potatoes are fork tender
Remove from oven and let stand 10 minutes
Sprinkle with parsley and serve