Preheat your oven to 180°C (350°F) and lightly grease or line a baking dish with parchment paper to prevent sticking and ensure even baking.
In a large mixing bowl, add almond flour, protein powder, eggs, Greek yogurt, olive oil, and sweetener, then mix everything together until a smooth and slightly thick batter forms without lumps.
Slice the apple thinly using a sharp knife, ensuring even slices so they cook properly and soften during baking.
In a small bowl, toss the apple slices with cinnamon and nutmeg so each slice is evenly coated with warm spices.
Pour the prepared batter into the baking dish and spread it evenly using a spatula to create a flat base.
Arrange the spiced apple slices on top of the batter, slightly pressing them into the surface so they stay in place while baking.
Place the dish into the preheated oven and bake for 25–30 minutes, or until the top is lightly golden and a toothpick inserted into the center comes out clean.
Remove from the oven and allow the focaccia to cool for at least 10 minutes before slicing, which helps the structure set properly.
Optionally prepare a simple glaze by mixing Greek yogurt with a small amount of sweetener and drizzle it over the top before serving.
Slice into squares and serve warm or at room temperature for the best texture and flavor.