In a food processor, combine the flours, cold whipped butter, Greek yogurt, maple syrup, and salt. Pulse until the mixture resembles coarse crumbs.
Add ice water one tablespoon at a time until the dough comes together.
Shape the dough into a rectangle, wrap it in plastic wrap, and refrigerate for at least 30 minutes.
Preheat your oven to 375°F (190°C).
In a large bowl, toss the sliced apples with lemon juice, sweetener, cinnamon, nutmeg, and cornstarch until well combined.
Roll out the chilled dough on a floured surface into a rectangle that fits your baking sheet. Transfer the rolled-out dough to the baking sheet.
Arrange the apple filling over the rolled-out dough, leaving a border around the edges.
Bake in the preheated oven for 25-30 minutes or until the crust is golden and the apples are tender.
In a small bowl, whisk together powdered sugar substitute, almond milk, and vanilla extract until smooth.
Once the slab apple pie has cooled slightly, drizzle the white icing over the top.
Allow the pie to cool completely before slicing into squares and serving.
This makes 8 large square slices. Each slice is 5 Points on Weight Watchers.