Pulse together flour, confectioners sugar, salt, and poppy seeds in a food processor.
Add butter and pulse until mixture resembles coarse meal.
Whisk together yolk, water, and lemon juice in a bowl, then add to processor and pulse until pea-size lumps form and dough holds together when squeezed. (Dough will appear slightly crumbly.)
Press 1/4 cup dough evenly into each tart shell with floured fingers (dough will be moist) and lightly prick bottoms with a fork. Chill until firm, about 20 minutes.
Put shells on a baking sheet and bake in lower third of oven until golden, 20 to 25 minutes.
Transfer to a rack and, when shells are just cool enough to handle, carefully loosen edges with a thin knife and remove from pans.
Fill each shell with 3 tablespoons lemon curd.
Beat together cream with confectioners sugar and lemon juice in a bowl with an electric mixer until it just holds stiff peaks.
Transfer to a pastry bag with a decorative tip and pipe onto curd (or dollop frosting with a spoon).
Shells may be made 2 days ahead, cooled completely, and kept in an airtight container
