Caramelized onions are the heart of a great patty melt. Heat the butter and olive oil in a large skillet over medium-low heat. Add the sliced onions and a pinch of salt. Stir occasionally and let them cook down until they’re golden brown and sweet, about 25-30 minutes. Patience is key here, but the payoff is worth it.

While the onions are doing their thing, mix all the sauce ingredients in a small bowl. Taste and adjust if needed – a little more mustard for tanginess or a dash of hot sauce for some heat. Set it aside; it’s going to work wonders later.

In a mixing bowl, combine the ground beef with onion powder, garlic powder, paprika, salt, and pepper. Divide the mixture into four equal portions and shape them into oval patties to match the bread slices.

Heat a skillet or grill pan over medium-high heat. Cook the patties for about 3-4 minutes per side, or until they’re cooked to your liking. Remove them from the heat and let them rest while you prep the bread.

Take your slices of bread and generously butter one side of each piece. On the unbuttered side, spread a layer of the secret sauce. Add a slice of Swiss cheese, a beef patty, caramelized onions, a slice of cheddar cheese, and top it off with another slice of bread (buttered side out).

Heat a clean skillet over medium heat. Place the assembled sandwiches in the skillet and cook until the bread is golden brown and crispy, and the cheese is melted – about 3-4 minutes per side. Press down gently with a spatula to get that perfect crispy crust.