Thinly slice all your onions and set aside.
3 large yellow onions
Heat up a deep pot over med - high heat.
Add butter and oil into the pot along with your thinly sliced onions.
2 tbsp butter, 1 tbsp oil
Stir for 5 minutes then reduce the heat to a medium low.
Stir occasionally to make sure it doesn't burn.
After cooking for 45 minutes to an hour it should be caramelized.
Separate your ground beef into three 120g balls. Smash them in between two pieces of parchment paper with a plate to make a thin patty.
360 g ground beef
Sear each patty over high heat for 2 minutes each side. Add a slice of American cheese and remove when melted.
3 slices American cheese
Toast your 2 slices of bread using mayo.
1 loaf white bread, mayo for bread
Mix together your sauce ingredients.
¼ cup ketchup, ¼ cup mayo, ¼ cup mustard, 2 tbsp pickle juice
Assemble your patty melt with sauce, caramelized onions, burger patties, more sauce, and caramelized onions.