First, wash the apple and lemon thoroughly. Then grate the apple, zest the lemon, and set aside.
Cream the butter and sugar in a bowl until light and fluffy. Gradually add the eggs.
Then add the yogurt, lemon juice, lemon zest, and grated apple.
In a second bowl, mix the flour with cornstarch, baking powder, and a pinch of salt. Combine the flour mixture with the creamed butter mixture using a hand mixer until smooth.
Pour the batter into a square baking pan (approximately 9x9 inches) and smooth the top. Preheat oven to 350°F.
While the oven preheats, prepare the crumble. Combine the rolled oats, cold butter, sugar, flour, lemon zest, and a pinch of salt with your hands until a crumbly dough forms. Sprinkle evenly over the cake batter.
Bake for about 40 minutes. Let cool completely and enjoy.
