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1
Heat the oven to 375F.
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2
Punch down and flatten the dough on a smooth, dry countertop.
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3
Use a dough cutter to divide the dough into 4 equal pieces.
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4
Use your fingers to gently stretch each piece of dough out into a mini pizza about 6 inches wide.
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5
Divide the onions equally among the rounds, plopping them in the center.
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6
Grab the scalloped potatoes from the fridge and cut into four 3-inch squares.
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7
Use an offset spatula to wrestle each square of potato gratin out of the baking pan and onto a dough round, directly on top of the onions.
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8
Take the edges of each dough round and pinch together to seal so that there is no speck of onion or potato in sight, then gently roll the ball between the palms of your hands to ensure the volcano has a nice, round, dinner rolly shape.
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9
Arrange the volcanoes, seam side down, 5 inches apart on a parchment-or Silpat-lined baking sheet.
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10
Whisk the egg and water together and brush a generous coat of egg wash on the buns.
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11
Use a paring knife to cut a 1-inch-long X in the top of each volcano.
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12
Divide the Gruyere evenly among the volcanoes, stuffing it into the X in each one.
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13
Bake the volcanoes for 25 minutes, or until the dough is a deep, golden brown and the Gruyere cheese on top is caramelized.
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14
They are best served warm; allow them to cool for at least 10 minutes before digging in.
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15
If youre saving a couple of volcanoes for later, let them cool, wrap them well in plastic, and store them in the fridge for up to 3 days; warm them in the oven before eating.
