Cook your bacon according to package directions and set aside.
Heat a large cast-iron skillet over medium-high heat. While the skillet is heating, season your patties generously with salt and pepper. Place your burgers on the skillet and cook the first side for about 3 to 4 minutes, or until the meat starts to brown. Flip and continue to cook for about 5 minutes more, or until the burgers are cooked to medium.
While the burgers are cooking, heat a large sauté pan over medium-high heat and add the jalapeños, onion, and mushrooms. Drizzle with 1 tablespoon of the melted butter and stir to combine. Cover the pan and let cook for about 10 minutes, or until the vegetables are soft and beginning to brown.
Once the burgers are done, remove to a plate and place a quarter of the vegetable mixture on top of each burger, then 3 slices of bacon and 2 slices of cheese atop each. Set aside and let the cheese start to melt on top.
Meanwhile, reheat the cast-iron skillet over medium-high heat. Brush the hamburger buns with the remaining melted butter and place them on the skillet for 1 to 2 minutes, or until the buns start toasting to a light golden brown.
Slather the buns with mustard and mayonnaise, then top the bottoms of the buns with lettuce, pickles, and tomato. Using a spatula, place each patty with the veggies, bacon, and cheese directly on top. Add your top bun and enjoy!