Preheat oven to 375 degrees. In a medium bowl, combine flour, oats, brown sugar, cinnamon, nutmeg, and salt. Stir until evenly combined. Cut butter into small cubes, then add to the flour mixture. Work the butter in with a pastry blender or with your hands until the mixture comes together into small crumbly clumps. Set aside.
In a large bowl, combine nectarines, blueberries, granulated sugar, tapioca, and vanilla extract. Stir to combine. Spread fruit mixture across a 2-quart baking dish, and sprinkle with the topping.
Bake for 45-50 minutes, until the fruit is bubbling and the topping is golden brown. Serve warm. With ice cream. Obviously!