Remove zest from lemons with a lemon zester or fine grater (Use the yellow portion of the peel and avoid any of the bitter white portions.) Juice lemons (about a cup). Combine lemon juice and zest with sugar in a heavy 2-quart saucepan.
Heat over medium heat until sugar is completely dissolved. Remove from heat and add butter, stirring until completely melted. Whisk in eggs.
Cook mixture over medium heat, stirring constantly until mixture thickens and coats back of spoon. Pour mixture through fine-mesh strainer; discard peel.
Chill curd before filling tarts.
Preheat oven to 350°F.
Combine flour and sugar. Cut in cold butter until mixture resembles a coarse meal. Add egg and mix until evenly moistened.
Divide mixture into small tart pans (twelve, 3-inch pans or six, 4½-inch pans). Press dough evenly over bottom and sides.
Place tart pans on baking sheet and bake 20 minutes or until lightly browned.
Let cool. Remove from pans, fill with chilled lemon curd, garnish with fresh fruit slices and serve.
Lemon curd is also delicious on scones or as a cake filling.