Preheat the oven to 425°F.

Prepare the onions: Trim the onion ends, then peel and discard the onion skins. Cut the onions through their “equator” into ½-inch-thick rings, keeping all the layers intact. Pour the oil into a rimmed half-sheet pan and place the onions on top. Flip each onion using a thin spatula and rub it in the oil so both cut sides get coated, careful to keep the slices intact. Sprinkle the top-facing side with thyme, salt, and pepper, then flip with a spatula and season the other side. Space the onions evenly apart on the pan.

Roast, part 1: Roast until the onions are nicely browned on the bottom, about 20 minutes. Combine the broth and balsamic vinegar in a measuring cup, then pour the liquid into the sheet pan, making sure it doesn’t reach higher than halfway up the onions (you don’t want them swimming in the broth!).

Roast, part 2: Continue roasting until about half of the liquid has evaporated, about 15 minutes. Remove the pan from the oven. Using a small offset spatula (or a butter knife), gently flip the onions, making an effort to keep the onions together. Return the pan to the oven, rotating it 90 degrees, and continue roasting until all liquid has evaporated, about 10 minutes more. Transfer to a serving platter and serve hot. Store leftover melting onions in an airtight container in the fridge and eat within 2 days. Love the recipe? Leave us stars and a comment below!