Preheat the oven to 425 degrees F.
Cut barely a sliver
off the bottom of each onion so they'll sit without rolling over. Pierce them in a few places with a sharp knife so they won't burst during cooking. Make 4 mounds of coarse salt in a baking dish
, and nest an onion in each. Bake the onions
15 minutes. Lower the heat to 325 degrees F and continue baking until the insides are almost applesauce
soft, about 1 1/2 hours, depending on the size of the onions.
To make the dressing: Whisk
together the yolk*, garlic, and vinegar, and season with salt, and black pepper. Beat in the oil, drop by drop, to make a thick dressing
. Taste, and add lemon juice
, if needed. Check the salt and pepper. Stir through the herbs
. Cover the vinaigrette
, and refrigerate until ready to serve.
To serve, set the onions on serving plates (leaving the salt mounds behind). Slit the onions open with a sharp knife, and spoon some vinaigrette into each until it starts bubbling up and out onto the plate. Serve the onions immediately.