Roll the pastry on a lightly floured work surface in one direction, turning clock wise after each roll into a circle about 0.3cm thick and slightly larger than a 26cm x 2cm loose-bottom fluted flan tin. Fold the pastry over the rolling pin and use to line the flan tin. Press the pastry into the flutes all the way round the tin. Trim the edges by rolling the rolling pin over the top of the flan tin, this will help to remove any excess pastry. Prick the base with a fork and chill in the fridge for 30 minutes.
To make the filling, heat the sunflower oil in a frying pan. Add the garlic and red onions and fry for 5 minutes or until softened and just turning golden. Remove from the heat and stir through the spring onions. Allow to cool. Preheat the oven to 180C/Fan160C/Gas Mark 4.
Place the chilled pastry case on a baking sheet. Line the pastry case with scrunched up then smoothed out baking parchment. Fill with either baking beans, rice or dried old beans to weigh it down. Place in the centre of the oven and bake blind for 20 minutes. Take the pastry case out of the oven and lift out the paper and baking beans. Brush the base and sides with the beaten egg and put back in the oven for a further 5-7 minutes or until the base is cooked and golden.
Mix the beetroot, peppers and nuts with the onion mixture and spoon half the mixture into the pastry case. Beat the eggs, cream and crème fraiche together in a large jug. Season with thyme and salt and pepper. Pour on top of the vegetables. Top with the remaining vegetables. Scatter the top with the feta cheese.
Place in the oven and cook for 50-55 minutes or until the filling is just set in the middle. Allow to cool for 10 -15 minutes. Remove the sides of the tin. Use a palette knife to slide the base away and place the tart on a serving plate. Serve with a little gem and watercress salad.