CRUST: In the bowl of a stand mixer fitted with a paddle attachment, cream butter and sugar. Add egg and mix until well combined. Add almond flour, all-purpose flour, baking powder, and salt. Mix just until dough comes together and there are no longer any dry streaks of flour. Dough will resemble cookie dough.
Scrape out dough and flatten into a small rectangle. Cover with plastic wrap and let chill in the fridge for 30 minutes.
Press dough into tart shell (with removable bottom). Use the flat bottom of a drinking glass or measuring cut to evenly spread and distribute dough into an even layer. Cover with plastic wrap and keep chilled in the fridge while you preparing filling.
Preheat oven to 350 degrees F.
FILLING: In a food processor fitted with blade attachment, combine almond paste, sugar, and butter. Pulse until well blended. Scrape down bowl as needed.
Add eggs one a time, pulsing until incorporated. Add vanilla, salt, baking powder, and flour. Process until mixture is smooth.
BAKE: Pour filling into chilled crust. Arrange nectarine wedges on top as desired. Sprinkle sliced almonds and turbinado sugar on top. Bake for 45-55 minutes until crust is well golden brown and the filling center no longer jiggles.