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1
Sprinkle yeast over warm water and stir in sugar. Let stand until frothy, about 5 minutes.
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2
Mix salt and flour in a stand mixer with the paddle attachment, then stir in egg and the yeast mixture to form a dough. Switch to a dough hook and knead on medium speed smooth and elastic (dough should pull away from sides of the bowl but stick to the bottom - add flour by the teaspoon until it tightens up), about 5 minutes. Form into a ball and let rest in a bowl on the counter, covered with plastic wrap, punching down with a wet fist every hour, for at least 2 hours and up to 3.
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3
Preheat oven to 500u00b0F with rack in middle.
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4
Turn out dough onto floured countertop, turning to coat, then flatten into a 7-inch disk. Toss together cheeses and press into a compact 3-inch ball with your hands. Place ball in middle of dough, then gather dough up around ball of cheese, squeezing excess dough together on top to enclose the cheese. Then begin to flatten ball into a disk, pressing down from the top and spreading cheese out from center, until dough is an 11-inch disk.
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5
Cut a 6-inch X through top of dough to expose cheese. Bake until golden and cooked through, with delicious melty cheese escaping, 12-15 minutes.
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6
Slice into wedges and serve.
