Preheat the oven to 300°F. Line the bottom and sides of an 8-inch baking pan with parchment paper, leaving enough overhang on all sides. Set aside.

For the crust: Combine the melted butter, sugar, vanilla, and salt together in a medium bowl. Add the flour and stir until everything is combined. Press the mixture evenly into the prepared baking pan. Bake for 15 minutes.

For the filling: Combine the cranberries, flour, granulated sugar, cinnamon, and nutmeg together in a large bowl until all of the cranberries are evenly coated.

For the streusel: Combine the oats, brown sugar, cinnamon, and flour together in a medium bowl. Cut in the chilled butter with a pastry blender until the mixture resembles coarse crumbs.

Remove the crust from the oven, increase heat to 350°F. Pour the cranberry mixture on top of the warm crust. Sprinkle the cranberry layer with streusel and bake for 30–35 minutes or until the streusel is golden brown.