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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.
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Sauté the onions until they are golden brown, stirring occasionally, about 20 minutes. If browned bits of fond build up on the bottom of the pan while caramelizing, deglaze with a touch of water.
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Once the onions are caramelized, add the sherry and reduce until dry. Season with salt and pepper to taste.
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Transfer the onions to a bowl and mix in the heavy cream, thyme and egg yolks.
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Roll the disk of dough out on a floured surface into a 10-inch circle. Transfer to a parchment-lined sheet tray.
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Pour the onion filling into the center of the tart dough and spread it leaving a 2-inch border around the edge.
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Sprinkle the onions with blue cheese and fold the extra crust over the onions.
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Bake until golden brown and bubbly, about 40 minutes.
