Adjust oven rack to middle position and preheat oven to 450°F (230°C). Line a rimmed baking sheet with aluminum foil.

Season chicken with 1 ½ teaspoons kosher salt and black pepper on both sides, and arrange skin-side up on prepared baking sheet. (For even better seasoning and crispier skin, let chicken stand at room temperature for at least 45 minutes, or refrigerate uncovered for up to 24 hours.) Bake for 15 minutes.

Meanwhile, in a medium bowl, toss onions with oil and ½ teaspoon kosher salt.

Once chicken has cooked for 15 minutes, working quickly, remove sheet pan from oven and arrange onions around chicken. Return to oven and continue to cook until onions are browned and chicken registers 175°F to 185°F (79 to 85°C) on an instant-read thermometer, about 15 minutes more.

Meanwhile, in a small saucepan, heat apricot jam, vinegar, Worcestershire sauce, mustard, Aleppo pepper, garlic, and remaining ½ teaspoon kosher salt over medium heat until simmering. Reduce heat to medium-low and continue to cook, stirring often, until thick enough to coat the back of a spoon, 2 to 4 minutes; set sauce aside.

Once chicken is cooked, remove sheet pan from oven and adjust oven to broil setting. Flip onions. Brush chicken and onions generously with about half (¼ cup) of the apricot glaze. Broil until chicken is crisp and charred in spots, about 5 minutes.

Remove from oven and use a brush to coat chicken with remaining apricot glaze.

Transfer to a platter, sprinkle with cilantro and Aleppo pepper, if using, and serve.