Heat the oven to 180°C fan / 190°C conventional.
Butter a baking dish. Add the apples and plums directly to the dish, then add the sugar, lemon juice, vanilla, cinnamon, cornstarch and salt. Toss everything together.
In a separate bowl, mix the flour, oats, sliced almonds, brown sugar and salt. Add the cold butter and vanilla, then rub together with your fingertips until you get large, irregular, buttery crumbs.
Scatter the crumble generously over the fruit.
Bake for 35–45 minutes, until deeply golden and the fruit is bubbling around the edges.
Let it rest for 10–15 minutes, then serve warm with a scoop of vanilla ice cream.