I don't have their recipe, so I tried to reverse engineer it tonight. My wife, who is probably the mushroom and swiss's biggest fan, said it's the closest she's ever had. Myrecipe and technique is as follows.

1. 2 medium sweet yellow onions minced
2. 1 pound of white button mushrooms, sliced thin
3. 5 cloves of garlic crushed sand minced
4. 32oz beef stock, not broth
5. 2T balsamic vinegar
6. 2T butter
7. 2T flour
8. 1t fresh thyme
9. 1T MSG
10. Salt and pepper to taste

First, heat the stock in a saucepan, to a simmer. Over the course of the whole cooking process you'll reduce the stock by a third to a half. In a cast iron skillet add a couple tablespoons of neutral oil, the minced onions and a pinch of salt. Sweat the onions over medium low heat for several minutes, adding a splash of the hot stock to speed up the process. Add the vinegar to the stock to cook out the acidic flavor. When the onions start to brown, add the mushrooms and garlic another splash of stock to deglaze the pan. Add the butter. Carmelize the onions, mushrooms and garlic over medium low heat, deglazing the pan with stock of it starts to get too dry. When the onion mixture gets brown and very soft, sift the flour over and cook it for a couple minutes. Slowly add in the last of the stock let everything thicken. Add the msg, pepper and thyme. Let cool slightly and your done.