Blend polenta, flour, salt and butter in a food processor until it resembles fine breadcrumbs. Add egg and 2-3 tablespoons water and pulse until the mixture comes together in large clumps, adding more water if necessary. Turn dough onto a clean bench and bring together into a ball. Wrap and refrigerate for at least 30 minutes

Roll out the dough between two sheets of baking paper and use to line a 27cm x 21cm loose-based rectangle tart tin. Line the tart case with baking paper and fill with baking weights or rice. Blind bake at 180°C for 20 minutes before removing the paper and weights and baking for a further 10 minutes, or until dry to touch. Cool slightly

Meanwhile, saute leek and tarragon in butter until soft and translucent, evenly spoon into the tart shell and top with zucchini ribbons. Whisk eggs and creme fraiche together, season to taste and stir in cheese. Pour mixture into the tart and top with asparagus and nuts. Bake for 25-30 minutes until golden and set. Cool for at least 10 minutes before removing from the tin and serve warm or at room temperature