JUICY TURKEY BURGERS WITH ONION JAM

Steps:

• Prepare the onion jam: Heat a large saute pan. Toss in the onion slices with the oil and a generous pinch of salt. Add the sugar and cook over medium heat until the onions are caramelized and tender, 20 to 25 minutes. Stir in the vinegar and if a splash of juice from the cornichon jar, if using, and cook 1 to 2 minutes. Set aside.
• For the burgers: Break up the meat in a medium bowl, spreading it up the sides of the bowl so that the seasoning really permeates the meat. Sprinkle with the Worcestershire and salt. Mix with your hands, but do not overmix. (The more you work the meat, the tougher the burger.) Gingerly form the meat into four round 1-inch-thick patties. Refrigerate for at least 20 minutes and up to 1 hour.
• Heat a large cast-iron skillet and add the oil. When the oil begins to smoke lightly, arrange the patties in a single layer at a distance from one another. Let cook, undisturbed, until the bottoms brown, 3 to 5 minutes. Turn the patties and let the bottoms brown again, 3 to 5 minutes. Lower the heat to medium-low, then add the stock to the skillet and cover. Simmer until the patties are cooked through and the internal temperature reaches 165 degrees F, 7 to 10 minutes. Remove the lid, shut off the heat under the skillet and place a slice of cheese on each patty. Let the patties rest with the heat off until the cheese is melted, about 2 minutes. Transfers the patties to a plate or cutting board and let rest for 1 to 2 minutes.
• Reduce the remaining liquid in the skillet by half, about 5 minutes, and serve it on the side of the burgers. People can spoon this liquid over the burgers instead of or in addition to ketchup. You can also just add this reduced liquid to the onion jam to enrich the flavor.
• Assemble the burgers: Arrange the bun bottoms on a work surface. Spoon some of the onion jam on each and top with one of the patties. Top with more jam and a few slices of cornichon. Top with the bun tops.