1. In a small bowl, combine the secret sauce ingredients. Stir, then set aside.2. Cut the potatoes into ¼" thick fries. Soak the fries in cold water for 30 minutes. Heat a couple of inches of vegetable oil in a deep, heavy pot to 325° F.3. In three or four batches, cook the fries for 4–5 minutes per batch. (You want the potatoes to be soft at this point, not browned.) Using a slotted spoon, remove each batch of fries from the oil and drain on paper towels. Increase the temperature of the oil to 400° F. Cook the fries again, in batches, until they are golden brown. Remove from the oil and drain on paper towels. Sprinkle with salt.4. Preheat a large skillet or griddle over medium-high heat. Divide the ground beef into four equal portions and shape them into balls. Season each with salt and pepper. Add oil to the preheated skillet or griddle. Once hot, place the meat balls onto the surface, leaving space between them.5. Using a sturdy spatula, smash each ball down until it reaches about ¼" thickness. Continue pressing down to create a good sear. Add sliced onions on top and cook for 2–3 minutes until the bottoms are browned and crispy. Flip the burger patties and cook for an additional 1–2 minutes. Add a slice of cheese and cook until melted. Remove from the heat and transfer to a plate.
6. In a separate skillet, heat some oil over medium heat. Add jalapeño peppers. Cook, stirring occasionally, until the jalapeños are slightly tender, about 4–6 minutes.
7. To assemble the burgers: Spread a generous amount of the secret sauce on the bottom half of each bun. Place a patty on top. Add a spoonful of jalapeños, add another patty, then cover with the top bun. Serve the burgers immediately alongside the fries. Enjoy!