Make the cream puffs: Preheat oven to 400°. Combine water, butter, granulated sugar, and salt in a medium saucepan. Bring to a boil, and remove from heat. Stir in flour until combined, and return to medium heat.
Dry the panade by stirring constantly for 4 minutes. It is ready when it pulls away from the sides and a film forms on the bottom of the pan.
Transfer to the bowl of an electric mixer fitted with the paddle attachment, and mix on low until slightly cooled, about 2 minutes. On medium speed, add 4 eggs, one at a time, incorporating each completely before adding the next. Lightly beat fifth egg, and add a little at a time until batter is smooth and shiny. Test by touching it with your finger and lifting to form a string. If string does not form, the batter needs more egg. If you’ve added all the eggs and the batter still doesn’t form a string, add water, 1 teaspoon at a time, until it does.
Using a pastry bag fitted with a plain ½-inch round tip, pipe ten 2½-inch rounds of batter onto parchment-lined baking sheets. Brush with egg wash, and sprinkle with sanding sugar. Bake for 15 minutes. Reduce temperature to 350°, and bake until puffed and golden and centers are dry, about 35 minutes more. Let cool on a wire rack.
Make the blueberry sauce: Bring 1 cup blueberries, the water, granulated sugar, lemon zest, and salt to a boil in a small saucepan. Reduce heat, and simmer until most of the berries have burst, about 5 minutes. Remove from heat. Add remaining cup blueberries, and crush lightly.
Make the lemon mousse: Lightly whisk lemon curd, and fold in whipped cream using a rubber spatula. Halve each cream puff, and fill bottoms with lemon-mousse. Spoon on blueberry sauce, replace tops, and serve.
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