INGREDIENTS
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2 big yellow onions, quartered and thinly sliced
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2 red onions, quartered and thinly sliced
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4 leeks, root ends and dark-green tops lopped off, leeks sliced 1/4-inch thin, and washed
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1 tablespoon unsalted butter
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1 tablespoon olive oil
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2 tablespoons unbleached all-purpose flour
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8 cups vegetable or chicken stock
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1 tablespoon Worcestershire sauce, [Editor’s Note: or a splash of fortified wine]
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1 teaspoon salt
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1/2 teaspoon freshly ground black pepper
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1 day-old baguette
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1/2 cup grated Parmiggiano-Reggiano cheese
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1/2 cup grated Gruyère cheese, plus more as needed