A Waitrose recipe and very enjoyable it was too.
300g shortcrust pastry
3 tbsp olive oil
2 medium onions, finely sliced
250g Swiss chard
2 medium eggs plus 1 medium egg yolk
200ml crème fraîche
Salt and freshly ground black pepper
50g finely grated Gruyère
Roll out the pastry on a lightly floured surface. Loosely wrap around the rolling pin and carefully unroll over a 25cm, round tart dish. Gently press the pastry into the dish and prick the bottom with a fork. Line with crumpled greaseproof paper and fill with baking beans. Chill for 30 minutes. Preheat the oven to 180°C, gas mark 4.
Put the pastry in the centre of the oven and bake blind for 20 minutes. Remove from the oven, remove the paper and baking beans and return to the oven for 5 minutes to dry out. Then remove from the oven. Keep the oven on.
Meanwhile, heat the oil in a frying pan over a low heat and fry the onion until soft. Prepare the Swiss chard by ripping the green leaves away from the white stems. Finely slice both and add the stems to the onions. Fry gently until they begin to soften then add the sliced leaves and fry until soft. Season to taste and tip the mixture into the pastry case. Beat together the eggs, egg yolk and crème fraîche, mix in the Gruyère, season to taste and pour into the tart, making sure that it seeps through the chard filling to the base.
Bake for 25 minutes until golden and slightly risen. Serve hot, warm or cold.