When I was coming up with the recipe for this burger, I thought of the charcuterie plate, where you have a few different components: mustard, a sweet-and-sour element, a pickled element, and maybe a salad. From that inspiration, we now have this beautiful burger with really nice Gruyère melted all over it and onion marmalade sandwiched in the middle. It's a game-changer.
Onion marmalade: Cut onions in half, then remove the core at an angle. Thinly slice the onion halves. Over medium heat, add the oil, followed by the onions. The onions will start to sweat and cook down, releasing their liquid. Reduce heat to low and cook, stirring every 2–3 minutes until the moisture is gone, 20 minutes or more as needed, depending on moisture level of the onions. When all of the moisture has been cooked out of the onions, turn the heat back to medium. Add the sugar, a pinch of salt, and the vinegar and bring to a boil to dissolve the sugar and combine. Turn heat to low and reduce until the mixture is a thick jam, about 45 minutes. Makes 2 cups.
Aïoli: Roughly chop 1 garlic clove. Sprinkle with 1/2 tablespoon of salt and press with the back of a knife to make a paste. Place a bowl on a damp cloth (to keep from slipping), and add the garlic paste, along with a squeeze of lemon and 1 teaspoon Dijon mustard. Whisk in 2 egg yolks. Drizzle the vegetable oil in a slow, thin stream, whisking the entire time until emulsified and creamy. Season to taste with salt and white pepper. Makes 1 cup.
Cook the meat: Form the ground chuck into a thin patty, taking care not to overwork the meat. Preheat a cast-iron skillet on high and add the oil. Season the burger with salt and pepper on both sides and sear in the hot pan, 2 minutes per side. Tilt the pan so the fat can circulate around the burgers.
Assembly: Preheat the broiler. While the burger is still in the skillet, spoon 2 tablespoons of the onion marmalade on top. Transfer patty to a baking sheet and top with 2 tablespoons of Gruyère. Repeat with remaining patties. Place the oven rack just below the broiler and broil the patties just until the cheese melts, about 45 seconds. On the bottom of the bun, spread 1 tablespoon of mustard, followed by the burger and a handful of arugula. On the top bun, spread aïoli, followed by pickles. Add the top bun. Repeat assembly for remaining burgers and serve immediately.