Prep: Preheat oven to 350°F (175°C). Line an 8x8 pan with parchment paper.
Make Crumble: Whisk flour, oats, sugars, baking powder, and salt. Cut in cold butter until crumbly.
Bake Crust: Set aside 1 ½ cups of the mixture. Press the rest into the pan and bake for 10-12 mins.
Mix Filling: Toss peeled, diced apples with flour, cinnamon, and nutmeg.
Assemble: Layer apples over the hot crust, then cover with the reserved crumble.
Bake: Bake for 30-35 mins until golden brown.
Cool: Let cool completely in the pan (crucial for slicing).
Glaze: Whisk powdered sugar, maple syrup, milk, and salt. Drizzle over bars and serve.