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Special equipment: a 3-quart baking dish
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Melt the butter and oil in a large Dutch oven or pot over medium heat.
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Add half the onions and celery, 1/2 teaspoon salt and a few grinds of pepper.
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Top with the remaining onions and celery, 1/2 teaspoon salt, a few grinds of pepper, the bay leaves and the thyme sprigs.
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Cook, uncovered and stirring occasionally, until all the liquid evaporates and the onions are very soft and golden brown, 45 minutes to 1 hour.
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(If the onions are soft but could be more golden, increase the heat slightly for a few minutes toward the end of cooking.)
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Add the flour, and stir until completely absorbed, about 1 minute.
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Add the broth and wine, and simmer until almost completely evaporated, about 2 minutes.
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Remove from the heat, and let cool for a few minutes.
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Remove the bay leaves and thyme stems.
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(The onions can be made, cooled and refrigerated up to 2 days ahead.)
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Meanwhile, preheat the oven to 350 degrees F. Bake the baguette slices on a baking sheet until they're slightly toasted but not golden, 2 to 3 minutes per side.
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Let cool.
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Shingle and slightly overlap half the bread slices in a 3-quart baking dish.
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Top with the onion mixture in 1 layer.
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Shingle, overlapping, with the remaining bread.
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Sprinkle the Gruyere over the top.
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Cover with foil, and bake until the onion mixture is hot and the cheese is melted, about 40 minutes.
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Remove the foil, switch the oven from bake to broil and broil until the top is golden in spots.
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Sprinkle the thyme leaves over the top.
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Serve hot.
