If using a pizza stone, and not a regular baking sheet, place pizza stone in oven and preheat to 350℉.
Place your pizza crust on a large plate, cutting board, or pizza peel if you have one, then evenly spread out the apple filling on top, leaving the outside, elevated, crust bare.
Then make the crumble topping by stirring together the flour, brown sugar, cinnamon, and oats. Cut in the cold butter with a butter knife or pastry cutter, until the butter is mixed, in clumps, with the rest of the ingredients.
Slide the ready to bake pizza onto your pizza stone, carefully, and bake for 20-25 minutes, or until the topping no longer looks dry, and starts to darken.
While the pizza is baking, stir together the powdered sugar, milk, and vanilla to make the icing.
Once the pizza is done in the oven, let cool slightly, then drizzle the icing over it before slicing and serving.