Preheat the oven to 350°F (175°C) and line a 13×9-inch baking pan with parchment paper.
Whisk the 2½ cups of flour with ¾ cup sugar (crust) and 1¼ tsp baking powder in a large bowl. Add the cold cubed butter.
Cut the butter into the flour mixture using a box grater or pastry cutter until the mixture resembles coarse crumbs.
Place one large egg in a small bowl and stir with a fork. Toss a portion of the butter-flour mixture with the egg, a few tablespoons at a time, until you form a damp, shaggy dough.
Press the dough evenly into the prepared baking pan, smoothing the surface with a spatula. Chill for 15 minutes or bake as the filling is preparing.
Beat the remaining 3 eggs (room temperature) in a medium bowl, then stir in 1 cup sugar (filling), 1 cup lemon juice, 2 tbsp lemon zest, and ½ tsp nutmeg until smooth and free of lumps.
Pour the filling over the chilled crust and return the pan to the oven. Bake for 20-22 minutes or until the filling is set and the edges just begin to brown.
Let cool to room temperature, then refrigerate for at least 2 hours before slicing. Serve with optional powdered sugar on top.