INGREDIENTS
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3 tablespoons butter
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3 onions, peeled and thinly sliced
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2 cloves garlic, chopped
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3 sprigs fresh thyme, leaves only
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1 tablespoon sugar
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1 teaspoon kosher salt
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1 teaspoon black pepper
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1 tablespoon white balsamic vinegar
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4 French rolls, split in half
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2 cups Gruyère or Fontina cheese, shredded