Combine the flour, oats, sugar and salt in a medium bowl. Add the cold butter and rub into the dry ingredients with the tips of your fingers. Do this until the butter is evenly distributed and there is no leftover dry ingredient mix. It should look like a pile of small to coarse blobs of streusel. Refrigerate until you’re ready to use it.
Make the filling:
Combine all ingredients for the filling in a large bowl and mix well.
Assemble the crumble:
Preheat the oven to 360°F. Transfer the filling to a 7x11 rectangular baking dish. Evenly top with the streusel topping, then sprinkle on the chopped almonds.
Bake for 35-40 minutes, or until the filling is bubbly and the topping is golden brown and crunchy. Allow to stand for 30 minutes so the sauce can thicken.
If you suspect your fruit to have a high water content, you might want to increase the flour in the filling to 2, or even 3 tablespoons. Indicators of this can be very early-season fruit, fruit that has grown for a long time/is very large or fruit that is already very ripe. Otherwise your filling might turn out too liquid.