INGREDIENTS
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2 tablespoons extra-virgin olive oil
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2 tablespoons unsalted butter
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3 1/2 pounds yellow onions, sliced
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Salt and black pepper
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1/3 cup dry white wine
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6 cups beef, chicken, or vegetable stock
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4 fresh thyme sprigs
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3 ounces gruyère cheese, grated
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4 slices sourdough bread