For the burgers, mix together all of the ingredients. Form into 6 patties, then chill in the fridge for 30 minutes.
Add 1 tsp oil to a non-stick skillet. Fry the burgers on the both sides until nicely browned and caramelised. Don't cook them all the way through as they will finish cooking in the sauce. Set aside.
In the same pan, add the butter and fry the mushrooms. When they are golden, add the garlic. Fry until fragrant, then add the stout. Reduce over high heat until halved in volume. Add the beef stock and reduce to half volume again. Add the vinegar, worcestershire sauce, salt, pepper. When reduced, add the cream. Bring to a simmer then return the burgers to the pan and baste with the sauce.
Reduce the sauce until thick and rich, basting and turning the burgers from time to time - about 10 minutes.
Serve in burgers with cheese and toppings of your choice!!!