Step 1

Preheat oven to 350°F. Whisk together sugar, eggs, egg yolks, and lemon zest and juice in a small saucepan over low until blended. Cook, whisking constantly, until mixture is thickened (when a line can be drawn down the back of a wooden spoon coated with lemon mixture), 7 to 8 minutes. Remove from heat, and add butter, 1 tablespoon at a time, stirring until butter is almost melted after each addition.

Step 2

Pour curd through a fine wire-mesh strainer into prepared tart shell. Bake in preheated oven until curd is set (filling will wobble), 8 to 10 minutes. Let tart cool completely, about 2 hours. Top with citrus segments, and serve.