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1
Melt the butter in a soup pot over low heat.
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2
Stir in the sliced onions, season with salt and pepper, and press a piece of buttered aluminum foil down on the onions.
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3
Cover the pan and sweat the onions over very low heat, stirring occasionally, 20-25 minutes.
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4
Remove the lid and foil, increase the heat, and continue cooking, stirring occasionally to prevent scorching, until the onions are reduced, concentrated, and very dark brown, 8-10 minutes longer.
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5
Meanwhile, char the halved onion.
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6
Heat an electric burner until hot, or heat a small saute pan over a gas burner.
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7
Sprinkle the cut surfaces of the onion with the sugar and set the halves, cut side down, on the burner or pan; cook until dark brown, 2-3 minutes.
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8
When the sliced onions are ready, stir in the broth and add the charred onion.
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9
Cover and bring to a boil; simmer to blend the flavors for 10-15 minutes.
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10
Make the baked croutes--cut the baguette into uniform slices, 3/8 to 1/2 inch thick; brush them lightly on both sides with melted butter or olive oil; put on a baking sheet and toast in a preheated 375 oven, turning once, until dry and lightly browned, 10-15 minutes.
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11
Discard the charred onion halves, taste the soup, and adjust seasoning.
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12
Preheat the broiler and reheat the soup if necessary.
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13
Put 4 to 6 deep flameproof soup bowls on a baking sheet and heat in the broiler.
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14
Set 2 or 3 croutes in each bowl and ladle the soup on top.
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15
Sprinkle with the cheese and broil until the tops are browned.
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16
Serve at once--onion soup must be scalding hot.
