The World's Finest
You will also need a 3¼ inch (8 cm) pastry cutter and two 12-hole patty tins with cups measuring 1¾ inches (4.5 cm) at the base, 2½ inches (6 cm) at the top and ¾ inch (2 cm) deep, well greased.
First make up the pastry. Sift the flour into a large bowl, and then add the butter. Take a knife and begin to cut it into the flour until it looks fairly evenly blended, and then add the Parmesan, mustard and a pinch of cayenne pepper, plus about 1½ tablespoons cold water to make a smooth dough, before discarding the knife and bringing it together with your fingertips. Then place the dough in a plastic food bag and put it into the fridge to rest for 30 minutes. In the meantime, pre-heat the oven to gas mark 4, 350°F (180°C).
After that, roll it out as thinly as possible, use the cutter to stamp out 24 rounds and line the tins with them. (The pastry will stand proud of the rim of the cups to allow for shrinkage.) Then prick the bases and brush with the beaten egg. Now bake on the middle and top shelves of the oven (swapping them over halfway through to ensure even browning) for about 10 minutes, or until the pastry is just cooked through, then cool them on a wire rack.
Meanwhile, for the filling, melt the butter in a large frying pan and cook the onions very gently, uncovered and stirring often, for about 30 minutes, or until they have turned a lovely golden brown caramel colour. Then leave to cool and set aside until needed.
Now whisk the egg with the cream and mustard in a jug and add some seasoning. Next, spoon a little of the onion mixture into each pastry case, spread it out evenly and pour the egg mixture over. Cut each cheese log into 12 thin slices (wiping the knife between slices to cut more cleanly; the cheese is quite soft, so you may have to reshape a few slices into rounds). Place a slice on the top of each tartlet, then top with a sprig of thyme and a sprinkling of cayenne pepper. Bake for 20 minutes, or until puffy and golden, swapping the tins again halfway through cooking.