Place your sliced plums into a large mixing bowl. Add the white sugar, lemon juice, and vanilla extract. Sprinkle the cornstarch over the fruit and stir gently.

Grease your medium baking dish lightly with a little butter. Pour the seasoned plum mixture into the greased dish. Spread the fruit out evenly so it cooks at the same rate.

Grab a separate bowl for your dry crumble topping. Combine the all-purpose flour, brown sugar, ground cinnamon, and salt. Mix them together quickly using a regular kitchen fork.

Add your cold cubed butter to the flour mixture. Rub the butter into the flour using your clean fingertips. Keep going until it looks like coarse bumpy breadcrumbs.

Stir the rolled oats into your crumbly flour mixture. Scatter this topping evenly over the plums in the dish. Cover all the fruit completely right up to the edges.

Preheat your kitchen oven to 180°C or 350°F. Place the assembled baking dish on the center oven shelf. Bake for about thirty-five minutes until the top is golden.

Check if the fruit juices are bubbling up nicely. Remove the hot dish safely using thick oven mitts. Let it cool down for ten minutes before serving.