• In bowl, sift in the flour and stir in the salt. Make a well in the centre and add the beaten egg, the cold water and the cubed butter. Get your hands in there and slowly incorporate all the flour and breaking down the butter. Soon you'll have crumbly dough so tip it onto the surface and gently knead it until it is fairly smooth and cohesive. Flatten it into a circle, wrap it in cling film and chill for an hour.

For the onions

• Carefully slice one onion into rounds, and reserve a quarter of the slices for the top. Slice the rest of the onions into thin strips. Melt the butter in a non-stick frying pan, and once hot and sizzling add the onion slices. Stir to coat in the melted butter, sprinkle with some salt and cook for 5 minutes as the water evaporates. Add the thyme sprigs.
• To really soften the onions, lower the heat and take a square piece of baking parchment, a little bigger than the frying pan. Scrunch it up then run it under cold water, squeeze off the excess then straighten it out. Place it over the onions to trap in the moisture. Leave the onions to cook for 10 minutes.
• Check on the onions - they will have browned on the bottom so stir them all together. You can leave them for another 10 minutes, or you can now add the balsamic vinegar and brown sugar. Increase the heat to let the vinegar and sugar caramelise then take the pan off the heat to cool. Remove the woody thyme stems.

For the pastry

• Once the pastry has rested for an hour, remove it from the fridge. Sprinkle some flour on the kitchen surface. Bash the pastry a couple of times with the rolling pin to loosen it up a bit as it will be quite firm. Roll it out into a big circle until it is all evenly the thickness of a £1 coin. Measure the tart tin on top to ensure there is enough overhang.
• Gently wrap one end of the pastry over the rolling pin then slide the tart tin underneath. Flip the pastry over so the side you were rolling is on the underneath - the outside of the tart. Rip off a corner of pastry dough and use it to press the edges into the flutes and corners. Smooth it down across the top, then roll the rolling pin over to cut away the excess on the sharp edges. Set the lined tin in the fridge for half an hour.
• Preheat the oven to 210°C/190°C fan/410°F. Place a tray in the oven to heat up.

For the filling

• Meanwhile, mix together the cream, eggs, nutmeg and some salt in a mixing bowl. Warm a saucepan with a little water and set the bowl on top. Make sure the heat is low and gently stir with a spatula. It will take a while for the mixture to warm but take your time, maybe listen to a podcast so you don't get too bored! It will thicken quickly so be ready to remove it from the heat (if necessary keep a sieve on standby!). Stir the onions into the custard, ensuring all the pieces are well coated.
• Take the pastry case out the fridge, pierce some holes in the base with a fork, then line with some crumpled baking parchment. Weigh it down with baking beans or uncooked rice. Place the tin on the hot tray in the oven and bake for 15 minutes.
• After 15 minutes, check the pastry by carefully lifting the baking parchment. If the base is still greyish, it will need longer but you can remove the parchment with the baking beans or rice. Return it to the oven for another 5 minutes, then brush it all over in egg wash and bake for 5 more minutes.
• Fill the baked tart case with the creamy onion filling, then top with the reserved circles of raw onion and some thyme leaves. Cut the goat's cheese into rounds and dot them across the top. Return the tart to the oven and bake for 20 minutes until the surface is golden and the cheese has melted.