Instead of grilled burgers, you and the kids will love this retro alternative. Refrigerate any leftover onions for future use.
Heat 2 tablespoons of butter and 2 tablespoons of vegetable oil in a medium skillet on high heat until it is melted. Add the onions and toss to coat evenly. Reduce the heat to low and the cook the onions for approximately 30-40 minutes, stirring occasionally to prevent burning. When the onions are uniformly browned, remove them from the skillet and set them aside. Season 12 ounces of ground beef with salt and pepper. Work it into the meat slightly with your hands and divide it into 4 equal patties. Grill or sauté the patties to the desired temperature: 2 minutes per side for medium rare, 3 minutes per side for well done. Set the cooked burgers aside.
Assemble the sandwiches: spread 1 side of each piece of bread with mayo. Add ½ slice of cheese, some caramelized onions, and a cooked burger. Top with another ½ slice of cheese and another slice of bread, mayo-side down. Once all of your sandwiches are assembled, heat a large cast-iron skillet or stovetop griddle, to medium-low. Butter each side of the assembled sandwich and place in the skillet. Cover and toast. When the bread is nicely toasted carefully flip each sandwich till the other side is done. Serve immediately with a side of ketchup drizzled with Sriracha, if desired. This makes a great summer lunch with a side salad.