Mix the flour and the salt with the sesame/poppy seeds. Add the cold butter in small pieces and rub it in with your fingertips. When the mixture resembles fine breadcrumbs, add the beaten egg and knead together, finishing with the well-chilled water. Bring together into a ball, cover with cling film and leave in the refrigerator.
Thoroughly grease your pie dish with butter. Roll out, turning the pastry occasionally to achieve an even result. Place the pastry into the pie dish, pushing it down. Cut off the extra pastry level with the top of the dish. Refrigerate for around 30 min.
Peel the butternut squash. Cut in half and remove the seeds in the lower part of the squash. Next cut it into cubes. Heat up a dutch oven with a steaming basket and put the pieces in the basket. Put on the lid and leave to cook for 20-30 min. Remove the steaming basket and put to one side. In a small bowl, beat the eggs, mustard and cream with a fork. Add a pinch of salt and a grind of pepper. Put to one side.
Take the pastry, prick with a fork, and cover with a piece of greaseproof paper/ baking parchment and pour dried vegetables such as lentils, beans or chickpeas on top, then pre-bake for 30 min in an oven pre-heated to 160°C. Once the pastry is pre-cooked, take out the paper and the beans brush the bottom of the tart with beaten egg and return it to the oven for 3 min. Remove the pie tin from the oven and adjust the oven heat to 180°C.
Place the cubes of cooked butternut over the surface of the tart. Pour the egg and cream mixture over the butternut, spreading it over evenly. Cut shavings of Roquefort and cover the surface. Sprinkle with ground nutmeg to taste. Scatter with flat chopped parsley.