1. Preheat oven to 180C.
2. For the pastry whiz flour and salt in food processor. Add chopped butter and whiz until mixture resembles fine bread crumbs. Add egg yolk and the water with motor running.
3. The pastry should resemble a ball. Take out of processor, form into dough ball. Flatten dough slightly to form a disc and wrap in cling film. Refrigerate while you make the filling.
4. Finely chop 1/2 an onion.
5. Melt the butter in a very large frying pan over medium/low heat. Add the onion and cook them, stirring regularly, for 5 minutes.
6. Meanwhile, thoroughly wash the chard, shake off excess water, and slice it (including the stalks) finely. Add it to the frying pan and cook, stirring occasionally, for 5 minutes or so until it’s wilted but the leaves are still deep green. (If there is liquid in the base of the pan, briefly increase the heat to boil it off.) Turn off the heat. Set aside.
7. Divide pastry into two even discs. Roll out pastry onto floured surface to approximately 3mm thick and line 2 pastry cases. Place foil in the pastry.
8. Whisk the eggs in a bowl to break them up, then whisk in the milk, cream, and a little salt, pepper and a pinch of nutmeg.
9. Spread the chard mixture evenly over the base of the tart shells. Sprinkle the grated cheeses evenly over this. Give the egg mixture another quick whisk in case it’s settled a bit and drizzle it over the cheese.
10. Bake the tart for 35-40 minutes, or until it’s deep golden and lightly set. To check, insert the tip of a fine knife into the middle and gently press the sides of the cut apart. The filling should be softly set with no liquid running into the cut.
11. Remove it from the oven and leave it to cool and settle for at least 10 minutes before serving. Then just slip off the outer ring of the tin, and gently slide the tart onto a serving plate.