Whisk together the secret sauce ingredients in a small bowl and set it to the side.
Heat the olive oil in a large skillet over medium heat. Add your thinly sliced onions and cook, stirring here and there, until caramelized about 15-20 minutes.
While onions cook, shape your ground beef into four patties. Season with salt and pepper.
Fire up another skillet on medium-high and cook the patties about 4-5 minutes per side for a good sear and ready doneness.
Once onions are done, spread secret sauce on one side of each rye bread slice.
Now assemble: place a beef patty on a bread slice, top with caramelized onions, then cover with another bread slice, sauce side down.
In your skillet, melt some butter then cook the assembled patty melts over medium heat until bread’s golden crispy about 3-4 minutes per side.
Remove from heat, slice each patty melt in half, and serve right away while it’s warm and melty.
Notes
Serve the patty melts warm with the secret sauce adding a tangy layer of flavor.