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1
For the soup: Preheat the oven to 400 degrees F. Slice the onions in half from root to tip, then slice them thin.
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2
Melt the butter in a heavy ovenproof pot or Dutch oven over medium-low heat.
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3
Add the onions, stir to coat them in the butter and cook, covered, until translucent and soft, about 20 minutes.
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4
Transfer the pot to the oven with the lid slightly ajar.
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5
Roast the onions for 1 hour, stirring twice to keep them from burning.
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6
(Some brown bits on the bottom of the pan are fine though.)
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7
Return the pot to the stovetop over medium heat (keep the oven on) and pour in the wine.
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8
Stir, scraping the pan.
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9
Cook until the wine reduces, 5 minutes.
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10
Next, add the beef broth, chicken broth, thyme, Worcestershire sauce and garlic.
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11
Reduce the heat to low and let the soup simmer for 45 minutes.
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12
For the croutons: Meanwhile, drizzle slices of French bread with olive oil and toast in the oven until golden.
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13
Set aside.
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14
To serve, turn on the oven broiler.
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15
Ladle the soup into ovenproof bowls.
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16
Place 1 or 2 pieces of the croutons on top of each serving of soup, depending on the size of the bowl.
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17
Place a thick slice of cheese on top of each bowl, then place the bowls on a baking sheet and broil just long enough for the cheese to become melted, bubbly and slightly toasted.
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18
Serve immediately.
