Position a rack in the center of the oven. Heat the oven to 425°F. Trim the root ends of the onions, leaving enough of the core intact to hold the onion wedges together. Trim the other end of the onions and cut the onions in half vertically. Cut each half into four wedges, about 1 inch wide.
In a medium bowl, combine the onions, potatoes, olive oil, and sherry vinegar and season with salt and pepper. Toss to coat well. Spread the vegetables and any liquid in the bowl on a large heavy-duty rimmed baking sheet in a single layer, making sure that the potatoes are all cut side down. Roast until the cut sides of the potatoes are crusty and golden brown, about 35 minutes. Turn the potatoes and onions using a metal spatula. Sprinkle the thyme leaves over the potatoes and return the pan to the oven and roast until the potatoes are very tender and the onions are very brown, about another 5 minutes. Taste and season with more salt if needed.
nutrition information (per serving):