Wipe mushrooms clean with a damp paper towel and slice.
In 2 Tbsp. of butter or oil, sauté mushrooms over medium heat until a little color forms.
Slice an onion and add to the pan, stir to coat with the butter/oil, and add a pinch of salt to draw the moisture out of the onions.
Continue cooking over medium heat, stirring occasionally to keep the onions from burning.
After about 20 minutes the color of the onions should begin to deepen. At this stage add Worchesteshire sauce and wine (or water if substituting) to deglaze the pan. Stir to remove the bits on the bottom of the pan. Continue to cook out the wine while you prepare your burgers.
Cook your seasoned burgers how you normally would; in a pan or on the grill. Top with Swiss Cheese and allow it to melt.
Top your burger with the caramlized onions and mushrooms. This can be served as is, or atop a hamburger bun.