01 - Pour vegetable oil into a deep fryer or heavy-bottomed pot to a depth of at least 3 inches. Heat to 350°F over medium-high heat.
02 - Rinse the cut potato fries in a large bowl of cold water to remove excess starch. Pat thoroughly dry with clean kitchen towels. Fry in batches for 4 to 5 minutes until lightly golden and just tender. Remove with a slotted spoon and drain on paper towels.
03 - Return the blanched fries to the hot oil in batches and fry for an additional 2 to 3 minutes until deeply golden and crisp. Transfer to a fresh layer of paper towels and season immediately with salt.
04 - While the fries cook, melt the butter in a large skillet over medium-low heat. Add the diced onions and cook, stirring frequently, until deeply browned and fully caramelized, about 12 to 15 minutes. Remove from heat and set aside.
05 - In a small mixing bowl, combine the mayonnaise, ketchup, yellow mustard, sweet pickle relish, white vinegar, sugar, and paprika. Stir until smooth and well blended. Refrigerate until ready to use.
06 - Arrange the crispy fries in an even layer on a baking tray. Lay cheese slices across the top. Broil on high for 1 to 2 minutes until the cheese is fully melted and bubbly.
07 - Scatter the caramelized onions evenly over the melted cheese. Drizzle generously with the prepared special sauce. Serve immediately while hot.