In a small heavy saucepan combine the peanut butter, the soy sauce, the lemon juice, the brown sugar, the garlic, 1/3 cup of the water, and the cayenne, bring the mixture just to a boil, whisking until it is smooth, and whisk in the scallion greens. Keep the sauce warm. In a small bowl whisk together the egg and the remaining 2 tablespoons water and add the shrimp. In a large plastic bag have ready the flour.