Adapted from “Bouchon,” by Thomas Keller
Makes about 1 cup.
1 pound (3 cups) Spanish onion, cut into ¼ ‑inch dice
1 cup extra-virgin olive oil
1 pound (about 6 medium) plum tomatoes
½ teaspoon minced garlic
1. Combine the onions, oil and a pinch of salt in an 8- to 9‑inch-wide saucepan and place over medium heat. As soon as the oil starts to simmer, reduce the heat to low and set the saucepan on a diffuser to maintain an even low heat. Allow the onions to stew very slowly and gently until the water has cooked away and the oil is clear again and the onions are golden brown, slightly darker than a golden raisin, about 2 ½ hours.
2. While the onions are cooking, cut the tomatoes lengthwise in half. Gently squeeze out the seeds and discard them. Grate the cut tomatoes, skin-side-out on the large holes of a box grater; the pulp will go through and the skins will stay behind, yielding about a cup of tomato purée. Discard the skins.
3. Add the tomato purée to the caramelized onions and cook until the water has cooked away and the tomatoes are sizzling in the oil, another 2 ½ hours. Stir in the garlic and another pinch of salt. The mixture may be kept, covered and refrigerated, for up to a week. The solids are lifted out of the oil, which should be reserved for another use.