* 1 large red onion
* 20g (3⁄4oz) butter
* 2 tsp olive oil
* 1 large egg
* 150ml carton double cream
* salt and freshly ground black pepper
* 15g (1⁄2oz) rocket leaves
* 25g (scant 1oz) Parmesan cheese, coarsely grated
* 25g (scant 1oz) mature Cheddar cheese, coarsely grated
For the filo pastry cases
* 25g (scant 1oz) butter, melted
* 2–3 large sheets shop-bought filo pastry
* 4 tsp fresh thyme leaves
4-hole Yorkshire pudding tin
1. Finely slice the onion. Heat the butter and oil in a large, non-stick frying pan until the butter has melted. Add the onion slices and fry over a medium–low heat for about 15 minutes, stirring only occasionally, until soft and caramelized. Remove the onion, drain on kitchen paper, and set aside to cool. Preheat the oven to 190ºC (fan 170ºC/375ºF/Gas 5).
2. Prepare the filo pastry cases: brush a little of the melted butter in the hollows of the Yorkshire pudding tin. Using a sharp knife, cut 16 squares, measuring 11 x 11cm (41⁄2 x 41⁄2in), from the filo sheets (how many you get from each sheet depends on the brand of filo).
3. Layer up four filo squares per hole, each at an angle to the previous squares, brushing melted butter and sprinkling thyme over each one. Scrunch up the pastry edges to add a bit of height.
4. In a small bowl, beat the egg, then stir in the cream and some salt and pepper. Roughly chop the rocket, setting aside just a few leaves for garnish. In a separate bowl, combine the two cheeses.
5. Carefully spoon the onion into the pastry cases. Scatter over half the cheese and the chopped rocket. Pour in the egg mixture, then top with the rest of the cheese.
6. Bake for 15–20 minutes or until the filling is just set and starting to turn golden brown at the edges. Remove from the tin and serve warm, garnished with the reserved rocket leaves.
Frying the onions in a mixture of butter and oil helps to stop the butter from overbrowning during the long cooking time needed for carmelisation to occur
Mary Berry's keys to perfection
1. Caramelise the onion: peel the onion. Using a sharp chef’s knife, quarter the onion lengthways. Cut the hard root from the base, then slice down each quarter lengthways into thin slices. This will give small, delicate pieces that are suited to the scale of the tartlet cases.
It’s important to fry the onion in a mixture of butter and oil. The butter adds flavour and the oil helps to stop the butter from overbrowning during the long cooking time needed to caramelize the onion. Fry the onion very slowly to bring out its sweetness. Stir occasionally to prevent it from burning, but not too often as you want it to start browning where it’s in contact with the pan.
As the onion starts to turn brown and gets a bit sticky, stir so it doesn’t burn and to bring the paler bits of onion to the bottom of the pan so they can also get brown. Scrape up the browned bits at the bottom of the pan using a spatula.
When the onion is done, it should be well reduced and evenly caramelized to a rich deep brown colour. Its flavour will become sweeter and more intense during the cooking process. Remove it using a slotted spoon and drain on kitchen paper
Ruffle up the edges of the filo to add a bit of height and texture
2. Make the crispy filo cases: stack the filo pastry sheets on a board and cut out the 16 squares, each 11 x 11cm, using a sharp knife in order to prevent the pastry from tearing as you cut. Filo pastry dries out extremely quickly and becomes very brittle, so keep the filo covered with a damp tea towel or cling film until you start lining the tins, which you
should do as soon as possible.
For each case, brush one square of filo with butter and lay it in a hole in the buttered tin; the edges of the pastry square should extend over the rim. Sprinkle with thyme. Repeat for the three remaining squares of filo, laying each at an angle to the previous ones so they overlap, and buttering and sprinkling thyme over each layer. Layering the filo like this strengthens the pastry cases.
To add a bit of height and interest to the pastry cases, ruffle up the edges. Use your thumb and forefinger to bring up the sides so they’re upright, then turn over the edges in small, soft folds, keeping the sides raised to maintain height. A bit of irregularity with the folds is fine and adds character to the cases. This paper-thin pastry is not easy to make, but it’s widely available in packets containing a number of ready-made sheets. Sizes of filo sheets can vary according to the manufacturer, so for this recipe you may need more than the number of sheets specified
More from Mary Berry:
Mary Berry Cooks the Perfect, published by DK, £25, dk.com. Also available to download on iBooks.