Method

• 1.
  Place the flour, sugar and 1/2 tsp salt in a bowl and stir to combine. Add the cold butter and using your fingers, crumble butter into flour until there are pieces no bigger than a dried bean. Using the palms of your hands, rub the butter and flour together to create thin sheets of butter (this will help to create thin flaky layers). Add 1/2 cup (125ml) iced water, 1 tbs at a time, until the pastry comes together. (You may not use all the water.) Turn pastry out onto a lightly floured work surface and knead until it just comes together. Shape into a ball, flatten into a disc and enclose in plastic wrap. Rest in the fridge for 1 hour.
• 2.
  Roll out pastry on a lightly floured work surface to 3mm-thick round and place on a baking tray lined with baking paper. Chill until needed.
• 3.
  Heat oil in a large non-stick frypan over medium-low heat. Add onion and 1/2 tsp salt flakes and thyme. Cook, stirring, for 15-20 minutes until soft and translucent but not coloured. Set aside to cool (you can make the onions up to 2 days ahead and store in the fridge).
• 4.
  Sprinkle 2 tsp plain flour over the pastry base and top with the onion, leaving a 5cm edge. Scatter over the chopped anchovy fillets, if using. Place the tomato in an even layer over the top and sprinkle with 1 tsp salt flakes and drizzle with extra olive oil.
• 5.
  Fold the edge of the pastry over the tomato. Lightly beat the egg and brush the crust with egg wash.
• 6.
  Preheat oven to 200°C.
• 7.
  Bake tart for 30 minutes to partially cook the pastry, then remove from the oven. Whisk creme fraiche and milk together in a bowl and drizzle mixture over the tomato. Sprinkle with the grated Gruyere and parmesan then return to the oven and bake for a further 20 minutes or until the cheese has melted and the edge of the tart is deep golden. Cool slightly before slicing. Serve with a green salad.