Preheat the oven to 200°C/Fan 180°C/Gas 6. Have ready a deep baking dish, about 27 × 23cm.
Tip the frozen fruit into the dish, add the apple chunks and spread out to cover the base. Sprinkle the sugar and spice evenly over the fruit. Dot the vanilla paste over the surface and sprinkle with the lemon zest.
To make the crumble topping, put all the ingredients into a food processor and pulse until the mixtures resembles crumbs. (Alternatively, put the butter, sugar and flour into a bowl and rub together with your fingertips until the mix resembles crumbs and then stir in the oats.)
Grab handfuls of the crumble mixture, squeeze together and then crumble the mix over the fruit in the dish – make sure the crumble covers the fruit completely. Bake on a high shelf in the oven for 35-40 minutes until the crumble topping is crunchy and golden and the fruit underneath is softened and saucy.
Remove from the oven and leave the crumble to cool slightly before serving. I like mine with lots of cold custard, but you can serve it with hot custard if you like, or vanilla ice cream or cream.