Preheat the oven to 375°F (190°C). Set a 9x6-inch baking dish on a parchment-lined sheet pan.
Add the sliced nectarines and frozen cherries to the baking dish. Sprinkle over the granulated sugar, cornstarch and salt, then add the almond extract. Toss until the fruit is evenly coated and no dry cornstarch remains.
In a medium bowl, stir together the flour, rolled oats, brown sugar, cinnamon and salt.
Pour in the melted butter and stir until the mixture is evenly moistened and forms large, irregular clumps.
Scatter the crumble evenly over the fruit without pressing it down.
Bake for 50–60 minutes, until the topping is deeply golden and the fruit is bubbling vigorously around the edges and through the centre. If the topping browns too quickly, loosely cover the dish with foil for the remaining bake time.
Cool for 30–45 minutes before serving. Serve warm with vanilla ice cream, if desired.