This is originally from Epicurious, but I modified it to use parchment paper because it worked better for roasting the onions. This is a wonderful side dish that pairs beautifully with most meals. It is also great warm or at room temperature.

INGREDIENTS

• 4 pounds medium-size red onions
• 1/4 cup olive oil
• 6 tablespoons (3/4 stick) butter
• 3 tablespoons sugar
• 6 tablespoons balsamic vinegar
• 1 tablespoon chopped fresh parsley
  

PREPARATION

1. Position 1 rack in center and 1 rack in bottom third of oven; preheat to 500°F. Line 2 large baking sheets with foil. Cut onions through root end into 3/4-inch-thick wedges. Place in medium bowl; toss with oil. Arrange onions, cut side down, on baking sheets. Sprinkle with salt and pepper. Roast until onions are brown and tender, rotating pans in oven and turning onions once, about 45 minutes.
2. Meanwhile, melt butter in heavy small saucepan over medium-high heat. Add sugar and stir until sugar dissolves. Remove from heat. Add vinegar. Return to heat. Simmer until mixture thickens slightly, about 2 minutes. (Onions and balsamic glaze can be made 1 day ahead. Cool. Cover separately and chill. Rewarm onions in 375°F oven about 15 minutes. Stir glaze over low heat to rewarm.)
3. Arrange onions on platter. Drizzle glaze over. Sprinkle with parsley.