Heat the oven to 375 degrees F and butter an 8x8-inch baking dish.
Melt the butter in a skillet over medium heat. Add the apples, 1 tsp cinnamon, ginger and nutmeg and cook for 4 to 5 minutes, until soft and golden at the edges.
In a large bowl, whisk the eggs, milk, yogurt, maple, vanilla, baking powder, salt and the remaining 1 tsp cinnamon.
Stir in the oats, then fold in most of the apples.
Pour into the dish, press the rest of the apples on top and scatter over the walnuts.
Bake for 32 to 36 minutes, until the center is set and the top is golden. Cool for 10 minutes, then cut into 6 squares.