Preheat oven to 400°F. 

Roll out the sheets of dough over a lightly floured surface. Cut 3-inch diameter rounds and place them onto the bottom and up sides of 12 ungreased muffin cups. Bake for 10 minutes, until golden brown. Remove from oven and allow the cups to cool down.

Prepare the lemon filling. In medium saucepan, whisk together sugar, starch, and salt. Gradually whisk in water and bring to a boil. Continue to cook over medium heat, stirring constantly for up to 1 minute to keep mixture from scorching the bottom of the pot and until it is shiny and thick. Remove from heat.

In a medium bowl, whisk egg yolks. Whisk about 1/4 cup of the hot sugar mixture into the egg yolks, then whisk the egg yolk mixture into the pot. Add lemon juice and lemon zest to the mixture. Cook over low heat, stirring constantly, for 3 - 5 minutes or until temperature reaches 160 F. Do not allow mixture to boil or it may curdle. Remove from heat and add the butter, mix until butter is well incorporated.

Allow filling to cool for 1 minute then pour into prepared crusts. Cover with plastic wrap and chill at least 1 hour before proceeding with topping. Top the tarts with mixed berries and arrange them in a platter resembling the colors of the flag. 

If desired, mix powdered sugar with milk until smooth. Drizzle over tarts to resemble the white stripes of the flag.