Preheat the oven to 190°C (375°F) with a rack in the centre. Have ready a 20 x 20 cm (8 x 8 inch) or similar baking dish.
Peel the apples if you like, core them and slice about 5 mm (1/4 inch) thick. Toss in the dish with the lemon juice, half the cinnamon, the maple syrup and cornstarch.
Melt the butter, then mix it with the oats, protein powder, chopped pecans, brown sugar and the rest of the cinnamon until it forms clumps.
Scatter the crumble evenly over the apples.
Bake for 35 minutes, until the fruit is bubbling at the edges, the apples are soft when pierced with a knife and the topping is golden. If the top browns too fast, cover loosely with foil.
Cool for 10 minutes so the juices thicken, then serve warm with the Greek yogurt.
Tips
A mix of Honeycrisp and Northern Spy gives sweet and tart.