adapted from One Bowl Baking by Yvonne Ruperti
Yield: 12 (4 X 2 inch) bars
2 cups rolled oats
1 1/4 cups flour
1 cup light brown sugar, packed
1 1/4 tsp cinnamon, divided
3/4 tsp salt
3/8 tsp baking soda (1/4 plus 1/8 tsp)
12 Tbsp unsalted butter, melted
5-6 (about 2 pounds) Granny Smith apples, peeled, cored, halved and sliced thin (about 1/8-inch thick), divided (I used a mix of Granny Smith and Fuji)
2 Tbsp granulated sugar, divided
1 Tbsp fresh squeezed lemon juice
Place the oven rack in the middle. Preheat to 375° F.
Combine the oats, flour, brown sugar, 1 teaspoon of the cinnamon, the salt, and the baking soda directly into a 13 x 9 inch baking pan. Add the butter and stir until moistened. Reserve 1 cup of the crumbs and set aside. Firmly press the remaining mixture into the bottom of the pan.
Spread half of the apple slices evenly over the crust. Sprinkle 1 Tablespoon of the sugar, the lemon juice, and the remaining 1/4 teaspoon cinnamon over the apples.
Layer the remaining apples and then sprinkle the remaining sugar over the top. Scatter the reserved oat mixture over the apples and cover the pan tightly with foil.
Bake for 30 minutes to start to soften the apples. Remove the foil and continue to bake until the oat topping is golden and the apples are tender, about 30 minutes longer.
Set the pan on a wire rack to cool. Cut into rectangles.