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 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, sandy crumbs.
Add the egg yolk and pulse just until the dough starts to form moist clumps. 3. Press the dough evenly into the bottom and up the sides of the prepared tart pan, aiming for about a ¼-inch thickness. Prick the bottom of the crust all over with a fork.
Freeze the crust for at least 30 minutes to firm up. 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 completely cool, spread the creamy yogurt mixture evenly over the bottom with an offset spatula. Carefully arrange the juice-kissed berries over the top until the cream layer is covered. 7.
Refrigerate the finished tart for 30 to 60 minutes to allow the filling to set slightly before slicing and serving.
Don't overmix the dough after adding the egg yolk. Pulse just until it comes together to avoid developing too much gluten, which can make the crust tough.
