In a large skillet over medium low heat, melt two tablespoons of butter then add the sliced onions. Caramelize the onions over about 30 minutes, stirring occasionally until the onions have browned and are tender. Remove from the pan.
In the same skillet, turn the heat up to medium then add the remaining butter, jalapeños, mushrooms. Saute until browned and softened.
Form the bison into four equal sized patties. Season with salt and pepper as desired. Heat a grill pan or grill to high and then set the burgers on the grill. Do not move the burgers for about 4-5 minutes to ensure perfect grill marks.
If using a grill pan, add a tablespoon of water to the pan and quickly cover the burgers to steam for 1-2 minutes. Then flip the burgers.
Let the burgers rest for a few minutes before serving. Top with the onions, mushrooms, and jalapenos, and other optional toppings like lettuce, tomatoes, and pickles. Serve right away and enjoy with your favorite Avatar!