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Heat the oven to 190C/170C fan/ gas 5. Arrange the plums cut-side down in a single layer in a roasting tin, sprinkle over the brown sugar and roast for 10 mins until tender but still holding their shape. Leave to cool a little in the tin.
Butter the base and sides of a 20cm springform tin and line with baking parchment. Tip the ricotta, caster sugar and lemon zest into a stand mixer and beat for 5 mins, or do this with an electric whisk. Beat in the eggs, one at a time, beating well after each addition. With the motor running, slowly pour in the butter, beating until fully combined. Add the flour and baking powder, and quickly mix just to incorporate – don’t over-beat. Spoon into the tin, then press the roasted plums on top, cut-side up, and sprinkle over the hazelnuts.
Bake for 50 mins-1 hr, or until a skewer inserted into the middle comes out clean. Leave to cool until just warm, or fully cool. Serve with crème fraîche.