Preheat the oven to 180°C / Fan 160 °C / Gas Mark 4. Place the plum halves in a pan with half the sugar, orange juice and zest. Cook gently for 10 minutes to soften.
Meanwhile, place the flour into a bowl and rub in the butter with your fingertips, until the mixture resembles fine breadcrumbs. Stir in the remaining sugar, flaked almonds and oats.
Divide the fruit between 4 dishes and spoon over the crumble mixture.
Bake for 20-25 minutes until golden and bubbling.