For the crust, stir together flour, salt and nutmeg in a medium-size bowl. Set it aside. In the bowl of a stand mixer fitted with the paddle attachment, beat the butter at medium speed until light and fluffy, about 2 minutes. Add the powdered and granulated sugars and mix until smooth. Stir the flour mixture into the butter mixture to form a thick pastey dough. Press onto the bottom of the baking pan in an even layer to form the crust. Place the pan in the oven and bake until *very* lightly golden around the edges, about 20 minutes.
While the crust is baking, you can prepare the filling. In the bowl of a stand mixer fitted with the whisk attachment, mix together the eggs, granulated sugar, lemon juice, lemon zest, and vanilla until smooth. Add the flour and mix until it is *just* combined.
Pour the filling into the pan over the baked crust and bake again until the edges are set but the middle is a little jiggly, about 15 to 20 minutes. Let it cool completely at room temperature before grabbing the parchment paper overhang and lifting the entire block out of the pan and onto a clean surface. Cover and refrigerate for 4 hours, then dust with the powdered sugar, cut into bars, and serve.