To prepare crust: Preheat oven to 350 degrees F. Coat an 8-inch-square or 7-by-11-inch baking pan with cooking spray.Advertisement
Stir flour, sugar and salt in a medium bowl. Stir in butter, oil and lemon zest until blended and crumbly. Press the dough in an even layer in the prepared baking pan. Bake the crust until firm to the touch, about 20 minutes.
To prepare filling: Whisk sugar, lemon zest and juice, egg, egg yolk and flour in a medium bowl until smooth. Pour evenly over the hot crust.
Bake until set, 15 to 20 minutes. Let cool completely in the pan on a wire rack, at least 2 hours. Cut into squares with a lightly oiled knife. Dust with confectioners' sugar just before serving.
Make Ahead Tip: Cover and refrigerate for up to 3 days.
1 carbohydrate, 1 fat