01 - Preheat oven to 350°F. Line an 8×8-inch baking pan with parchment paper, extending edges for easy removal.
02 - In a mixing bowl, combine flour, sugar, and salt. Cut in cold butter using a pastry cutter or fingers until mixture resembles coarse crumbs. Press evenly into the bottom of the prepared pan. Bake for 12 minutes until lightly golden.
03 - In a bowl, whisk together ricotta cheese, egg, granulated sugar, vanilla extract, and lemon zest until a smooth mixture forms. Evenly spread over the warm crust.
04 - In a skillet over medium heat, melt 1 tablespoon butter. Add diced apples, granulated sugar, cinnamon, and nutmeg. Cook for 5 to 7 minutes until apples are tender but retain their shape. Remove from heat and allow to cool slightly.
05 - Spread the cooled apple filling evenly over the ricotta layer in the pan.
06 - Mix flour, brown sugar, and cinnamon in a bowl. Stir in melted butter until large crumbs form. Sprinkle evenly over apple layer.
07 - Bake for 25 to 28 minutes, or until the topping is golden and the center is set. Allow to cool completely in the pan before cutting into bars.