Heat the oven to 200C. Toss the rhubarb, apples, vanilla and sugar together in an ovenproof dish. Squeeze the juice of one lemon over the top and roast for 10 minutes.
Mix the flour and ginger together in a large bowl. Rub in the butter with your fingertips to create a chunky breadcrumb textured mixture. Stir through the sugar and porridge oats.
Sprinkle the crumble topping onto the fruit and cook for a further 30-35 minutes or until the topping is lightly golden brown. Serve with ice cream, custard or cream.
