Place a pizza stone or an inverted rimmed baking sheet on the bottom rack of the oven and preheat to 500 degrees F.
Combine the crab, cream cheese, water chestnuts, mayonnaise, soy sauce, Chinese hot mustard, sesame oil, scallions, garlic, 1 teaspoon salt and a pinch pepper in a large bowl. Set aside.
Dust a piece of parchment paper lightly with flour. Stretch or roll out the pizza dough into a 12-inch round on the parchment, then transfer the dough on the parchment to a pizza peel or another inverted rimmed baking sheet. Spread with the crab mixture. Whisk the egg with 1 tablespoon water in a small bowl. Brush the crust with the egg wash and sprinkle with the sesame seeds.
Slide the pizza on the parchment onto the pizza stone or baking sheet and bake until the crust is golden, 12 to 15 minutes.
Drizzle the pizza with duck sauce and top with the chow mein noodles and more sliced scallions. Serve with more Chinese hot mustard.
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