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Preheat the oven to 375 degrees.
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To caramelize the onions, heat a sauté pan over medium heat and add the olive oil and butter. Sauté the onions until they are nicely caramelized, stirring occasionally, about 20 minutes. If browned bits form on the bottom of the pan while the onions are cooking add a splash of water to deglaze.
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Once the onions are fully caramelized to a rich golden color, deglaze with the sherry wine and season with salt and pepper to taste. Remove from the heat.
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While the onions cook, roll the disk of dough out on a floured surface into a 12-inch circle. Transfer to a 10-inch tart pan. Fold in the edges and trim even with the rim of the pan. Chill until you ready to bake the tart.
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Stir the heavy cream, egg yolks and thyme into the onions.
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Remove the tart crust from the refrigerator, spread the onions evenly over the bottom, and top with the crumbled blue cheese.
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Bake on a parchment-lined sheet tray until golden brown and bubbly, about 40 minutes.
