Layer Sliced Onions & Mushrooms in bottom of 10x15 Baking Dish.
In large bowl combine Hamburger, Onion Soup Mix, Garlic, Parsley, Milk, Bread Crumbs & Worcestershire Sauce.
Form mixture into patties with your hands. I make mine about baseball size and flatten to about 1/4" inch.
Preheat oven to 350
Brown patties on both sides in frying pan on medium high heat. As you pull them out of frying pan, place in baking dish on top of Onions & Mushrooms. (Try to keep a single layer, but depending on patty size you may have to offset them on top of each other a little)
Once all patties are in baking dish, drizzle a little of the grease from the frying pan over the top.
Cover with foil and bake for 30-40 minutes (unitl meat juices run clear and onions have softened)