Preparation Preheat oven to 350°F with rack in middle. Pulse onions in 3 batches in a food processor until very coarsely chopped (there may be a few large pieces remaining), transferring to a bowl. Repeat with shallots, then garlic. Heat oil in a deep 12-inch heavy nonstick skillet over high heat until it shimmers, then sauté onions, shallots, and garlic (stir often) until golden and browned in spots, 25 to 30 minutes Grind fenugreek seeds in grinder or with mortar and pestle. Add to onion mixture along with remaining ingredients, 1 tablespoon salt, and 1 teaspoon pepper and stir until combined. Transfer to a parchment-paper-lined large 4-sided sheet pan and spread as thinly and evenly as possible. Bake, stirring occasionally with a skewer to separate onions, until well browned and barely moist, 1 to 1 1/4 hours. Cooks' note:Vadouvan keeps in the refrigerator 1 month (cool before covering) or in the freezer 6 months. This recipe's headnote has been updated as a part of our archive repair project.