Onions are too often the supporting actors in the kitchen when they should be the stars.
For the onions:
For the glaze:
1 Prepare the onions: Preheat the oven to 400F. Slice off any straggly roots from the onions, leaving the firm root centers intact. Cut the onions in half from tip to root. Peel off the skins until all the papery layers are removed. Cut each half into 4 wedges, slicing through the root so that the wedges stay intact.
2 Pack onions into baking dish: Pack the onions as tightly as you can into a 1 to 1 1/2 quart baking dish (9-inch round or oval, 8-inch square).
Sprinkle the olive oil, balsamic, 1 tablespoon of the oregano, salt, pepper evenly over the top. (No need to stir or you'll disturb the pretty arrangement!)
3 Roast the onions: Cover the baking dish with foil and roast the onions for 30 minutes. Remove the foil and continue roasting the onions uncovered for 30 minutes more.
4 Make the glaze: In a bowl, whisk the balsamic, mustard, salt, and pepper. Gradually whisk in the olive oil.
5 Pour the glaze over the roasted onions and return the dish to the oven. Continue cooking another 10 minutes, or until the onions are very tender when pierced with the tip of a knife.
6 Sprinkle the onions with the remaining 1 teaspoon oregano before serving.