In a large stainless steel skillet, add the butter and the onions. Turn the heat to medium, cover, and cook, stirring regularly, until the onions are softened and starting to brown, about 7-9 minutes.

Uncover, turn the heat to low, and continue to cook, stirring often, until the onions are very soft, sweet, and a deep brown color. If the onion starts to stick at any point, add a splash of water and scrape the bottom of the pan to deglaze. I usually have to do this 3-4 times throughout the process.

You can remove the onions in as little as 30 minutes, or cook them for an hour or longer. They will continue to get softer and sweeter as you go. I take mine out around the 45 minute mark. Season to taste with salt.

Deglaze the onion pan. Melt the butter in the skillet and add the mushrooms.

Let them cook WITHOUT stirring, 4-5 minutes, or until golden brown on one side. Flip and continue to cook until browned on both sides. Add the garlic and saute for about a minute, then season to taste with salt.

Heat a cast iron skillet over medium high heat. Add the patties and cook until browned on the bottom, about 4-5 minutes. Flip over, and cook an additional 3-4 minutes or until it reaches an internal temperature of 165 degrees. Add the cheese for the last 2 minutes of cooking.

To assemble, spread the mayonnaise on the buns (if using). Top each one with a burger patty, caramelized onions, and mushrooms. Add greens if desired. Serve.