Heat the oven to 450ºF. Stir 1 tablespoon fresh thyme leaves, ¾ teaspoon kosher salt, and ¼ teaspoon black pepper together in a small bowl.

Prepare 1 ½ pounds small or medium yellow onions: Trim about ¼-inch from the top and bottom of each onion. Peel the onions. Halve or cut each onion into thirds so the rings stay intact (¾- to 1-inch thick).

Place the onions on a 9x13-inch rimmed baking sheet (quarter sheet pan) or 9x13-inch metal baking pan (avoid glass, as it could shatter). Drizzle with 2 tablespoons olive oil, then use your hands to coat the onions all over with the oil. Season all over with thyme mixture. Arrange in a single layer (they can be touching).

Roast until the bottoms are just starting to brown, about 20 minutes. Meanwhile, stir ¾ cup low-sodium chicken broth and 2 teaspoons apple cider vinegar together in a liquid measuring cup or small bowl. (This is a good time to make the breadcrumb topping.)

Carefully flip each onion round with a thin, flat spatula and roast until the second side is light golden-brown, 10 to 15 minutes.

Pour in the broth mixture and roast until the onions are very tender, about 20 minutes more. Transfer to a serving platter and drizzle any remaining liquid in the pan over the onions. Sprinkle with the breadcrumb topping.

Heat 1 tablespoon olive oil in a small skillet over medium heat until shimmering. Add ⅓ cup panko breadcrumbs, ½ teaspoon fresh thyme leaves, and ¼ teaspoon kosher salt. Cook, stirring often, until golden-brown, 2 to 3 minutes. Remove the skillet from the heat.