- In a large skillet, melt 2 tablespoons of butter over medium-low heat. Add the onions and cook over low heat, stirring occasionally, until they are golden and soft – about 30 minutes.
- Make the sauce by combining the mayonnaise, ketchup, mustard, garlic powder, and black pepper. Set aside.
Heat a large cast-iron skillet over medium-high heat. Add two tablespoons of butter then cook the burger patties according to the package instructions. Wipe the skillet clean.
- Assemble each sandwich by spreading the sauce on one side of all 8 slices of bread. Divide the onions between 4 slices of bread. Top each with a burger patty, one slice of each cheese, and then another slice of bread.
- Add 2 tablespoons of butter to a skillet over medium-low heat. Swirl to coat the pan with the melted butter. Add the assembled sandwiches and cook until the bread is golden brown. Flip the sandwiches and add the remaining 2 tablespoons of butter. Move the sandwiches around to coat them in butter. Cook until golden brown. Serve immediately.
Recipe courtesy of Stacey Little.