Preheat the oven to 180°C/350°F/gas 4.
Place the apples and berries in a large pan over a medium heat. Add a few tablespoons of water. Cook for about 5 minutes, until the fruit is soft. Stir in the maple syrup and set aside.
Meanwhile, make the topping. Place the oats and butter in a large bowl. Use your fingertips to rub the butter into the oats, until the mixture looks like breadcrumbs. Mix in the pecans and seeds.
Pour the fruit into a large ovenproof dish. Sprinkle the crumble mixture over the fruit. Bake for about 35 minutes, until the crumble is golden and the fruit syrup begins to ooze up the sides of the dish. Spoon the crumble into warmed serving bowls and top with a dollop of Greek yoghurt.