Fry K&N's Burger Patties as per instructions on pack.
Assemble burger by layering the cooked patty with lettuce on the burger bun, with a large dollop of mushroom sauce.
Garnish with chopped black olives and parsley.
Serve with fries and coleslaw.
In a saucepan, heat the butter over medium heat until it becomes frothy.
Add the onions and garlic, and sauté until they are caramelized.
Add mushrooms and cook for about 5 minutes or until tender.
Add the demi-glace sauce, bring to a boil and lower heat to a simmer and cook for about 10 minutes.
Top with freshly chopped parsley and serve immediately.