How to Make It
Preheat the oven to 325°. Peel off the outer layers of the onions until each one is about 3 inches in diameter. Trim the root ends so the onions stand upright. Arrange the onions on a large rimmed baking sheet and cover with foil. Bake for about 4 hours, until they are partially softened.
Remove the foil and bake the onions in the upper third of the oven for 2 hours longer, rotating them occasionally, until they are very tender and richly browned outside. If the onions start to pop out of their skins, push them back in.
Meanwhile, spread the chopped olives on a microwave-safe plate. Microwave at high power for 12 minutes until the olives are dehydrated but still plump; transfer to a food processor and pulse until finely chopped. Transfer the olives to a medium bowl and wipe out the food processor.
Pulse the almonds in the food processor until finely ground; transfer to a rimmed baking sheet and toss with the demerara sugar. Bake in the same oven as the onions for about 6 minutes, stirring a few times, until golden and toasted. Let cool. Add the almonds to the olives and toss well.
In a saucepan, bring the demiglace to a boil. Transfer to a blender. Add the oil and vinegar and blend well. Season with salt.
Using a serrated knife, cut off the top fourth of each onion. Transfer the onions to plates. Spoon the vinaigrette on top and garnish with the almond-olive mixture. Serve right away.
Demiglaces are available at specialty shops and from dartagnan.com.