This is a fantastic tart to make for entertaining – packed with caramelised onions and delicious cheese, and encased in crisp, enriched shortcrust pastry. It can be made the day before and reheated on a baking tray if you are pushed for time on the day. I used our homegrown onions, which we have recently been harvesting and drying by the wood fired oven, and thyme from the garden for this. They are simple ingredients, but onions slowly fried in butter create the most incredible flavour.
Start by making your pastry, which you can do either by hand or using a food processor. First cut your butter into small cubes of roughly 1cm using two knives to avoid touching it with your hands, and place it in a large mixing bowl.
Sift your flour and salt from a height into the bowl, to get the air into the ingredients. Gently rub the butter into the flours using your fingertips. A light touch here is important; lift up the flour as you do this and let it fall back into the bowl to create a lighter texture for the mix. You should end up with a breadcrumb-type consistency – don’t overwork it. If using a food processor, pulse the flour and butter together until you reach this consistency.
Mix 2 tablespoons of water with a beaten egg yolk, and sprinkle it evenly over the mixture. Use a blunt kitchen knife or palette knife to incorporate it as quickly as possible and bring the dough together. If you need more water add it gradually and sparingly – you only need enough to bind the dough. If your ingredients look dry then add some more, but stop before it becomes wet and tacky; you want to end up with a soft dough. If using a food processor, it can be a little harder to tell as you can’t touch the dough, so stop the motor from time to time to check the consistency. If you push your ball of dough together and it cracks, the gluten needs a little more working.
Shape your dough into a disc and wrap it in cling film. Rest it in the fridge for at least 30 minutes.
To blind bake your pastry case, preheat your oven to 220°C. On a lightly floured work surface use a rolling pin to ridge the dough and start to push it outwards before you roll it. Rotate at each quarter turn, and when it is big enough, roll it out into a circle shape, 2-3mm thick. Lay the pastry over the tin using your rolling pin to help lift it, and use your fingers to press it snugly into the grooves. Trim off any excess pastry. Prick the base of the pastry with a fork to prevent bubbles forming. Cover the base and sides of the case with baking parchment, and weigh it down with baking beans. Make sure the beans are pushed right into the edges of the tin.
Blind bake the pastry case for 10 minutes, until pale golden with no wet patches of dough. Remove the parchment and beans and bake for a further 10 minutes until just golden brown, then remove from the oven and set aside.
When you are ready to make your filling, reduce the oven heat to 180°C.
Heat a knob of butter for frying in a heavy bottomed saucepan. Fry the onions for 45 minutes over a medium-low heat, with a pinch of sugar (optional) and a pinch of salt. Stir from time to time to prevent sticking. When they are almost meltingly soft and starting to caramelise, remove from the heat.
Using a pestle and mortar, grind the mustard seeds into a fine powder. Mix the crème fraîche, eggs, ground mustard seeds, thyme, and three-quarters of the cheese together in a large bowl, and season with black pepper. Pour into the tart case, and sprinkle the remaining cheese on top. Bake for 30 minutes, until a gorgeous golden yellow in colour. Leave to sit for 5 minutes or so before serving warm.