Ingredients
Equipment
Method
Bake the potatoes
- Heat the oven to 400°F.
- Scrub and dry the potatoes, prick each several times with a fork, and rub with the oil and 1 teaspoon kosher salt.
- Place the potatoes on a rimmed baking sheet and bake for 50 to 65 minutes, until a knife slides easily into the centers.
- Rest the potatoes for 5 minutes.
Make the spread
- Stir the mayonnaise, ketchup, relish, and vinegar together in a small bowl.
- Refrigerate the spread until serving.
Cook the onions
- Melt the butter in a 12-inch skillet over medium-low heat.
- Add the onions and 1/4 teaspoon kosher salt, and cook, stirring every few minutes, for 25 to 30 minutes, until deeply golden and soft.
- Transfer the onions to a bowl.
Cook the beef
- Season the beef with the remaining 1/2 teaspoon kosher salt and the pepper.
- Form the beef into 4 patties about 1/2 inch thick.
- In the same skillet over medium-high heat, cook the patties in 2 batches for 3 to 4 minutes.
- Spread the mustard over the patties' uncooked tops before flipping.
- Cook for 2 to 3 minutes more, until each patty reaches 160°F.
- Transfer the patties to a plate and rest for 3 minutes.
Assemble
- Split each potato lengthwise and fluff the centers with a fork.
- Place 1 slice of cheese in each potato, then top with the beef, onions, pickles, and spread.
Notes
For fluffy centers, bake the potatoes until a knife slides easily into them. Refrigerate leftovers in an airtight container for up to 3 days; freezing is not recommended because the potatoes and toppings may turn watery. For a gluten-free meal, use certified gluten-free ketchup, relish, and mustard.
