To make the mushroom confit, melt butter in 1 tablespoon of the oil over medium-high heat in a large saute pan. Add onion and saute until they begin to turn translucent. Add mushrooms and cook until they give up their juices. Lower heat and cook until the mixture caramelizes, then season with salt and pepper to taste and remove to a bowl. Cover and set aside.
In the same pan, heat the other tablespoon of oil over medium-high heat and sear the steaks about 2 minutes each side. Reduce the heat to low and add the ale. Maintain the ale at a simmer, just below boiling and cook uncovered until fork tender, about 45 minutes to 1 hour. Remove the steak to a carving board to rest. Increase the heat under the skillet to allow the ale to reduce by two-thirds. Season with salt and pepper, remove from heat and whisk in butter just before service.
Slice steak into thin slices and arrange on a platter. Spoon mushroom onion confit over and spoon sauce around.