In a food processor combine the flour, hazelnuts, and cinnamon with sugar and blend until the nuts are finely chopped. Add the cold butter and pulse until fine breadcrumbs. Add the beaten egg yolk and pulse until the mixture comes together and starts to form a ball.
Transfer to bench top and gently knead together for a few seconds to form a smooth ball. Wrap and chill the pastry for 30 minutes.
Pre-heat the oven to 185°C. Roll out the pastry between two sheets of baking paper until about 26cm. Carefully transfer the pastry to a 23cm tart tin and press evenly over the base and up the sides of the tin, trim neatly and chill for 15 minutes.
Line the pastry with baking paper and pastry weights or beans and blind bake for 12-15 minutes until pastry is just starting to colour around the edges, remove the paper and weights and cook a further 5-10 minutes until evenly golden. Brush the pastry with egg wash as soon as it is out of the oven to seal the crust. Allow to cool before filling.
Meanwhile, whisk the mascarpone with the caster sugar and orange zest until smooth, chill until required.
Place the rhubarb, orange juice, and honey in a medium saucepan and simmer gently until just tender. Add the strawberries and cook a further minute. Set aside to cool.
To assemble the tart, spoon ¾ of the mascarpone mixture into the cold pastry shell and smooth over the base. Spoon over the honey, orange rhubarb and strawberry mixture.
Before serving, top with the remaining mascarpone and garnish with fresh flower petals.