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Heat the oven to 190C/170C fan/gas 5. For the base, oil a 20cm tart tin and line the base with baking parchment. Pulse the oats in a food processor until coarsely ground. Add the rest of the ingredients along with a pinch of salt and pulse to combine. Firmly press the mixture into the bottom and sides of the tin. Bake for 8-10 mins until golden brown. Leave to cool completely.
Meanwhile, make the berry layer. Put the blueberries, citrus zest and juice, maple syrup and 150ml water in a small saucepan and simmer for 15 mins over a medium heat until the berries are soft. Blitz to a purée using a hand blender or food processor, then pass through a sieve and return to the pan (you should have 300ml liquid). Dissolve the agar in 100ml warm water, add to the pan and simmer for 2 mins. Leave to cool for 10 mins, then spread over the pastry base. Chill for 1 hr until set.
For the filling, put the blueberries, syrup and lemon zest in a small pan and cook for 5 mins over a medium heat until soft. Blitz using a hand blender or food processor, then pass through a sieve. Return to the pan along with the milk, vanilla, agar (dissolved in 4 tbsp warm water), cornflour and a pinch of salt,
and whisk to combine. Cook over a medium heat for 2 mins, then cool, stirring occasionally. Pour over the cooled tart. Chill for 5 hrs or overnight. Decorate with more berries, lime zest and rose petals, if you like. Will keep chilled for two days.