Because these caramelized onions are so deeply flavored, any style of extra-virgin olive oil is suitable for this recipe. Be patient while cooking the onions. Long, slow cooking renders them sweet with a lasting flavor. You can prepare them ahead, if you wish.
And do use your best balsamic vinegar. The intensity of balsamic vinegar varies; add it drop by drop until it imparts a complexity to the onions, but not enough so they taste of vinegar.
6 tablespoons extra-virgin olive oil, divided
2 large onions (about 2 pounds), peeled, halved, and thinly sliced
fine sea salt
balsamic vinegar to taste
4 slices grilled or toasted bread
Fleur de sel and freshly ground black pepper
Heat 4 tablespoons of the oil in a large skillet over high heat until it trembles, becomes aromatic, and easily coats the bottom of the pan. Add the onions and turn the heat to very low. Sprinkle with salt. Cook the onions uncovered, stirring occasionally, until they are the color of a polished mahogany table, about 1 hour. They will shrink dramatically.
Transfer the onions to a bowl and let them cool. Add balsamic vinegar drop by drop until the flavor of the onions is complex but not vinegary. Sprinkle with fleur de sel if needed.
Brush the bread with the remaining 2 tablespoons oil.
Put a generous pile of onions on each slice of bread. Add a few grindings of pepper and serve.
YIELD: 4 servings
SOURCE: Recipe courtesy of Fran Gage, author of The New American Olive Oil