1. Heat olive oil in a large pan over medium-low heat. Add the sliced onions and caramelize them by cooking for twenty to twenty-five minutes, turning regularly, until they are very brown. Season with a little salt and pepper to taste.
2. While waiting, heat a cast-iron skillet over medium-high heat. Melt 2 tablespoons of butter in the heated pan.
3. In a large bowl, carefully mix the ground beef, ketchup, Worcestershire sauce, 1/2 teaspoon of onion powder, 1 teaspoon of salt, and a pinch of pepper. Divide the meat mixture into four equal portions and shape each into a patty slightly wider than the bread slices.
4. Brown the patties well by cooking them in the heated skillet for four to five minutes on each side. Remove from the skillet and set aside.
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