2 pounds (4 medium) red onions, sliced into ¼-inch rings
¼ cup balsamic vinegar
Kosher salt, to taste
Directions
Preheat the broiler on high. On a sheet pan, arrange the onion rings in a single layer. Broil until charred completely, 9 to 11 minutes.
Let cool slightly, then transfer to a food processor and pulse, slowly drizzling in the vinegar until a thick but coarse jam forms. Season with salt and use immediately or store in a sealable container for up to 1 week.