Preheat the oven to 350 degrees F.
In a medium bowl, combine the flour, baking powder, baking soda, and salt. Stir together and set aside.
In a separate large bowl, beat the eggs and sugar together for about 15-20 seconds with a hand mixer. Add in the applesauce, Greek yogurt, buttermilk, oil and vanilla extract. Mix together until smooth.
Add the dry ingredients into the wet and stir until just combined. There will still be lumps! Fold in the 1 cup grated apple.
Pour half of the batter into a greased 9-inch pie dish, spring form pan or square baking pan. Spread out the batter. Lay a thin layer of the sliced apples over the batter. I do about 2 layers of the thinly sliced apples.
To make the cinnamon sugar, mix the sugar and cinnamon together until combined. Sprinkle of the thinly sliced apples.
pour the remaining coffee cake batter over the sliced apples and gently spread out.
To make the crumb topping, combine the flour, brown sugar, cinnamon, pinch of salt, and nutmeg into a medium bowl. Stir until fully combined. Using a pastry cutter or fork, cut the cold butter pieces into the flour mixture until mixed in.
Sprinkle the crumb topping over the coffee cake batter. Bake for 28-32 minutes or until the coffee cake is set and toothpick comes out clean.
Let cool slightly and enjoy!