Start by heating the 2 tablespoons of butter in a skillet over medium heat. Once melted, add the sliced onions and sprinkle with salt and pepper. Cook, stirring occasionally, until the onions are golden brown and crispy, about 10-15 minutes. Remove from heat and set aside.
In the same skillet, increase the heat to medium-high. Season the burger patties with salt and pepper on both sides. Cook them for about 4-5 minutes on one side, then flip and place a slice of blue cheese on top of each patty. Cook for an additional 3-4 minutes or until the burgers reach your desired doneness.
While the burgers are finishing up, slice the burger buns in half and toast them in the skillet until golden brown. This adds a nice crunch and helps prevent sogginess.
Start with the bottom half of the toasted bun. Spread 1 tablespoon of sweet and spicy mustard on each bun. Layer with a burger patty topped with melted blue cheese, followed by slices of fresh tomato, a handful of baby arugula or kale, and a generous portion of crispy fried onions. Finish with the top half of the bun.
Place the assembled burgers on a serving platter and dig in immediately while they are hot and delicious. Pair with your favorite sides for a complete meal.