For the tomato sauce, heat the extra virgin olive oil in a saucepan, add the onion and sweat on a medium heat until softened.
Reduce the heat to low, then add the tomatoes, basil leaves and salt and simmer for 25 minutes, stirring from time to time.
Should the sauce appear dry, add a little hot water.
Preheat the oven to 400F.
You want the white stalks and only the tender inner leaves of the Swiss chard.
(The discarded harder green leaves can be used in another recipe or cooked as above.)
Wash well and dry, then dust with flour.
Beat the eggs, stir in 2 tablespoons grated Parmesan, the 1/2 tablespoon flour and a pinch of salt and pepper.
Dip the chard in the beaten egg.
Pour enough olive oil in a large frying pan to cover the base generously, then place over a mediumhigh heat.
When hot, fry the chard on both sides until golden.
Remove and drain on kitchen towel.
Line a 8 x 8 1/2-inch ovenproof dish with some of the tomato sauce, then arrange a layer of the chard on top, followed by some of the mozzarella, Parmesan and basil leaves.
Continue layering like this until you have used up all the ingredients, finishing with a cheese topping.
Bake for about 35 minutes, until golden brown.
Switch off the oven and leave the Parmigiana to settle in the oven for 10 minutes before serving sprinkled with extra grated Parmesan.