Heat olive oil in a pan and your mushrooms. Add a pinch of salt and cook until golden brown. Transfer in a bowl and set aside.
In a pan heat your olive oil, add onion and sweat for 5 minutes. Add garlic and sweat for another 2 minutes.
Add cognac or red wine vinegar (I prefer red wine) and reduce until most liquid is gone. Add stock and reduce by half. Add your cream and simmer for 5 minutes. Add your thyme and stir.
Spread your peppercorns over your work station. Form patties with sirloin and press down on peppercorns for each side.
Heat pan with olive oil and cook your patties to your liking.
Top patties with mushroom and cheese, cover to melt cheese.
Let your patties rest for about 4 minutes. Serve on bun and spoon over your sauce and boom! Burger au Poivre!