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1
In a large wide pot, melt the butter over medium heat.
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2
Add the onionsto keep them from stewing rather than browning, spread them in as close to a single layer as possible.
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3
Cook, stirring occasionally, until they are very brown and soft, about 30 minutes; they will need more frequent stirring toward the end.
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4
Stir in the wine and bring to a boil, scraping the bottom of the pan vigorously with a wooden spoon to get up all of the browned bits.
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5
Continue scraping while pouring in the beef stock.
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6
Then pour in the chicken stock.
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7
Season with the salt and pepper.
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8
Bring to a boil.
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9
Lower the heat and simmer for 10 minutes.
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10
The soup can be made ahead to this point and refrigerated for up to 3 days.
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11
To gratine, preheat the broiler and, if necessary, return the soup to the simmer.
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12
Lay 2 slices of toast in the bottom of each ovenproof bowl or crock, or make two layers of bread in a large earthenware casserole.
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13
If using, mix together the Gruyere and Parmesan.
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14
Pour the soup over the slices of bread.
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15
Sprinkle the cheese mixture evenly over the soup.
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16
Or, if using the mozzarella, lay 4 thin slices over the top of each serving or all the slices over the entire contents of the casserole.
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17
Place the containers under the broiler for 3 to 4 minutes until bubbling and brown.
