Real home cooking. You can only get a tart this good if you do it yourself from a home kitchen. Taking plain old onions and ordinary eggs, this dish magics them into something wonderful.
- Make the pastry. By hand: Sift the flour and salt into a large bowl. Cube the butter. Rub it lightly into the flour between your fingertips until it resembles fine breadcrumbs. Finely grate the cheese and stir into the mix with a fork. Add the water gradually, mixing with the fork for a pliable dough that’s neither dry nor sticky (add more flour or water to get it right). Roll the dough lightly into a ball then flatten it into a disk. Wrap in clingfilm and chill for 1 hour. By machine: pulse the flour, salt, cheese and butter until fine. Add the water gradually, pulsing between additions until a dough forms. Stop and test with your fingers to check it’s not too dry. Roll, wrap and chill.
- Preheat the oven to 180c. Sit the pastry on a floured board. Grease a 23x3cm loose-based fluted tart/quiche tin. Roll the pastry out in a circle to fit the tin. Without stretching the pastry, roll the pin under it and lift it over and down into the tin. Press into the base and sides for a close fit without it cracking. Use spare pastry to fill any holes/ breaks, leaving a slight overhang on the edges in case it shrinks. Prick the surface lightly with a fork.
- Cut a piece of greaseproof paper and lay it into the pastry to protect the base and sides. Fill it with baking beans and bake blind for 10-20 minutes, till the sides are hard and lightly coloured. Remove the paper and beans and brush lightly with egg white. Bake again for 4 minutes. Allow to cool if you have time. Increase the temperature on the oven to 190c.
- Meantime, make the filling by thinly slicing the onions. Melt the butter with the oil in a large pan on a low heat. Cook the onions with the garlic and a pinch of salt very slowly for 20-30 minutes until soft and translucent. Remove and allow to cool until needed. Pour off any excess moisture.
- Once the onions and base are ready to go, combine and beat the whole eggs, egg yolks, sour cream, double cream, Dijon mustard and seasoning. Grate the cheese over the base and top with the onion mix, then sprinkle with thyme leaves. Carefully pour over the sour cream mix and bake for 30-40 minutes until puffy and golden.
- Remove and allow to settle for 10 minutes. Remove from tin.
- Sit the tart on a plate or a board. Cut into slices at the table. Serve with various salads and cold meats.