BURGER Patties: Combine ingredients. Shape mixture into 16 (5.3 oz.) patties (or into 4 patties for trial recipe).
MUSHROOM Sauce: Sweat shallots and garlic in oil. Add mushrooms; cook until softened. Add parsley, A.1., wine and stock. Bring to simmer; cook until sauce begins to thicken. Remove from heat; slowly stir in butter until melted. Keep warm until ready to use.
FRIED Onions: Combine flour and seasonings in bowl. Coat onions in flour mixture, shaking off excess. Soak onions in buttermilk 10 min.; drain.
FRY in 350ºF deep fryer until golden brown and crispy. Drain well.
FOR each burger: Grill 1 Burger Patty to desired doneness; top with 2 cheese slices.
FILL 1 bun with prepared burger, 2 Tbsp. Mushroom Sauce, 4 Fried Onions and 1 Tbsp. A.1.