balsamic roast onion

4 lbs medium-size red onions, peeled
1/4 cup olive oil
6 tablespoons butter
3 tablespoons sugar
6 tablespoons balsamic vinegar
1 tablespoon chopped fresh parsley
Directions
1Position 1 rack in center and 1 rack in bottom third of oven; preheat to 500°F.
2Line 2 large baking sheets with foil.
3Cut onions crosswise nearly, but not quite, all the way through. Make two or three more sectional cuts - always stopping before you cut through, so that onion is in 3/4-inch-thick wedges (the onion stays attached at one end and resembles an onion 'flower' (with very thick petals ;-) ).
4Place in medium bowl; toss with oil.
5Arrange onions, cut side down, on baking sheets.
6Sprinkle with salt and pepper.
7Roast until onions are brown and tender, rotating pans in oven and turning onions once, about 45 minutes (they will caramelize and be quite dark in parts but, if you have coated them well with olive oil, will not have a 'burnt' taste).
8Meanwhile, melt butter in heavy small saucepan over medium-high heat.
9Add sugar and stir until sugar dissolves.
10Remove from heat.
11Add vinegar.
12Return to heat.
13Simmer until mixture thickens slightly, about 2 minutes.
14(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.) Arrange onions on platter.
15Drizzle glaze over.
16Sprinkle with parsley.