Combine ground beef, onion soup mix, and egg in a large mixing bowl, and combine. Form into about 8 mini slider patties.

Melt butter in a large frying pan and add the butter, sliced mushrooms, sliced onion, Worcestershire sauce, salt, and pepper. Cook over a medium heat, until the onions are translucent and the mushrooms are cooked.

Grill the slider patties on the bbq or cook in a large frying pan over medium heat. Once fully cooked, add a slice of swiss cheese to each burger patty so it can melt.

To assemble the mushroom Swiss sliders, cut open a brioche bun, add the burger, top with sautéed mushrooms and onions, and add about a teaspoon of HP steak sauce or to taste. Enjoy