• Pre-heat the oven to 200ºC/400ºF/Gas mark 6

For the plums

• Put the plums, sugar and mixed spice into a saucepan over a low heat, stirring until the sugar has dissolved. This should take no more than 10 minutes
• Spoon the mixture into a large, oven-proof casserole dish. Set aside while you make the crumble mixture

For the crumble topping

• In a large mixing bowl, combine the flour, oats, hazelnuts, sugar and salt
• Rub in the cold butter until the mixture is lumpy
• Turn out the mixture on to the spiced plums and spread evenly over the top. It's not important to cover the entire surface
• Bake for 45 minutes to an hour - until the topping begins to turn a golden brown colour