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Heat the oven to 180C/160C fan/gas 4 and line a 20cm square tin with baking parchment. Put the flour, brown sugar and butter in a food processor, and pulse until it comes together into a dough. Transfer to a bowl and stir in the oats, then press most of the mixture into the tin (reserving some for the top) in an even layer over the base and partway up the sides. Put the reserved dough in a small bowl and stir through the pecans, if using.
Mash the raspberries, jam and a squeeze of lemon juice together in a bowl using a fork, then stir through the plums and spread the fruit mixture over the base. Scrunch the reserved dough and pecan mixture between your fingers to create a crumble texture, then scatter this over the fruit. Bake for 50-55 mins, or until the fruit is bubbling at the edges and the crumble is golden. Leave to cool completely in the tin, then slice into squares and serve. Will keep in an airtight container for up to a week. If you like, reheat in a low oven to serve warm.