Preheat your oven to 350°F (175°C). Lightly coat a 9-inch tart pan with a removable bottom with cooking spray.
Spread the almonds, pecans, and hazelnuts on a baking sheet and toast in the oven for 12 to 15 minutes, until fragrant and lightly browned.
Let cool completely. 2.
In a food processor, pulse the cooled nuts, whole-wheat flour, sugar, and salt until the nuts are finely ground.
Add the diced, chilled butter and pulse until the mixture resembles coarse crumbs.
Add the egg yolk and pulse just until moist clumps form. 3. Press the dough evenly into the bottom and up the sides of the prepared tart pan. Prick the bottom of the crust all over with a fork.
Place the pan in the freezer to chill for 30 minutes. 4.
Adjust the oven temperature to 400°F (200°C).
Bake the chilled crust for 12 to 14 minutes, or until it's firm and golden brown.
Let it cool completely in the pan on a wire rack. 5.
While the crust cools, prepare the filling.
In a small bowl, whisk together the sour cream, Greek yogurt, brown sugar, vanilla extract, and orange zest until smooth.
In a separate bowl, gently toss the blueberries and raspberries with the orange juice. 6.
Once the crust is cool, spread the creamy sour cream mixture evenly over the bottom. Carefully arrange the berries over the cream filling.
Chill the tart in the refrigerator for at least 30 to 60 minutes before serving to allow it to set.
Don't skip freezing the crust before baking. This crucial step solidifies the butter, which helps prevent the sides from slumping down as it bakes.
