1. Preheat oven to 350°.
2. Rinse the nectarines and berries. Cut nectarines off their pits into 1/2-inch-thick wedges and drop into a large bowl. Add the berries, granulated sugar, and lemon juice and mix gently just until combined.
3. In another bowl, stir the brown sugar, flour, rolled oats, vanilla, cinnamon, nutmeg, and salt until well combined. Add the butter and cut it in with a pastry blender or rub it in with your fingers until the mixture resembles coarse meal. Stir in the pecans.
4. Spread fruit mixture level in a shallow 2- to 2 1/2-quart baking dish. Sprinkle oat-pecan mixture over the top.
5. Bake until juices at the edges of the baking dish are bubbling and the top is crisp and golden, 40 to 45 minutes.