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1
Melt the butter in a large Dutch oven or heavy soup pot over medium-low heat.
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2
Add the onions, thyme, bay leaf, and measured salt.
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3
Slowly melt the onions, stirring frequently until the onions are very soft and a deep golden brown, about 45 minutes (if, at any point, the onions appear to be getting too dark, or beginning to stick to the pan, reduce the heat).
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4
Add the flour, stir well to combine it with the onions, and cook, stirring, for 1 minute.
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5
Add the wine and simmer, stirring, for 2 minutes.
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6
Add the broth and water, bring it to a boil, and simmer gently, stirring occasionally, for 30 minutes.
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7
Taste and season with additional salt, if necessary, and pepper.
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8
Heat the oven to 350 degrees F and arrange a rack in the middle.
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9
Arrange the baguette slices in a single layer on a baking sheet and toast for 8 minutes.
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10
Turn the slices over and toast an additional 7 minutes.
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11
Turn on the broiler and arrange the crocks on a rimmed baking sheet or in a shallow baking pan.
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12
Pick the bay leaf and thyme sprigs out of the soup and discard them.
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13
Ladle the soup evenly among the crocks, then top each with a baguette toast.
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14
Sprinkle the crocks with the shredded Gruyere and grated Parmesan.
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15
Broil, about 4 to 5 inches from the heat, until the cheese is melted and bubbly, about 2 minutes.
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16
Serve immediately.
