Preheat your oven to 375°F (190°C). In a medium bowl, melt 1/2 cup of butter, then mix in 1 cup of rolled oats, 1/2 cup of all-purpose flour, 1 teaspoon of cinnamon, and a pinch of salt. Stir until the mixture resembles coarse crumbs. Set aside.

Peel, core, and slice 4 medium Granny Smith apples thinly. Toss the apple slices in the juice of half a lemon to prevent browning. In a large skillet over medium heat, combine the apples with 1/2 cup of sugar, 1 teaspoon of cinnamon, and 1/4 cup of water. Cook for approximately 10 minutes until the apples are slightly softened.

Stir in a cornstarch slurry made from 2 tablespoons of cornstarch mixed with 2 tablespoons of cold water into the apple mixture. Cook for an additional 2 to 3 minutes until it thickens and becomes syrupy. Remove from heat and cool briefly.

Roll out the prepared sourdough crust on a floured surface. Transfer the crust to a baking sheet lined with parchment paper. Evenly spread the warm apple filling over the crust, leaving a small border around the edges. Sprinkle the prepared oat topping over the apple filling.

Bake for about 20 minutes until the crust is golden brown and the topping is bubbly. The aroma of cinnamon will fill your kitchen.

Remove the pizza from the oven and let it cool on the baking sheet for at least 10 minutes before slicing.