This classic easy-to-make rustic dessert uses plums with the addition of a little bit of crystallized ginger in the filling. If you don’t have the ginger or don’t like it, feel free to omit it. The topping is baked ahead of time, which helps crisp it up and prevents it from becoming soggy. While the topping is baking, you can prepare the filling.
Preheat the oven to 350°F. Line a rimmed baking sheet with parchment paper or a silicon baking sheet.Combine the flour, oats, both sugars, salt, cinnamon, ginger, and nutmeg in a large bowl. Stir vigorously with a balloon whisk until all the ingredients are evenly distributed.
Stir the melted butter and vanilla together in a small bowl or glass measuring cup. Drizzle the liquid over the dry ingredients, tossing to coat them, until all the liquid is absorbed and the dry ingredients start to clump together.
Spread the mixture in an even layer onto the baking sheet. Bake for 15 minutes. The edges should JUST start to darken but there shouldn’t be much change in color.
While the topping is baking, make the filling by cutting the plums into 1-inch thick slices into a large bowl. Gently toss with 4 tablespoons of sugar, cornstarch, vanilla, as well as the lemon zest and crystallized ginger if you are using. If your plums are particular tart and sour, add a tablespoon or two of sugar to help balance it. Grate the peeled apple with the hole from a larger grater into the bowl and mix in.
Pour the filling into a 9-inch deep dish pie plate or another 2-quart baking dish. Once the topping has baked, pull it out of the oven. Once it is cool enough to handle, sprinkle the crisp topping over the fruit.
Place the crisp in the oven and bake 50 to 60 minutes or until the fruit filling is bubbling and thick. Let cool to room temperature for the filling to thicken up before serving.