Cook the potatoes in lightly salted boiling water for about 10 minutes, until tender and drain thoroughly.
Vary the filling as you wish, using cooked pasta instead of potatoes, and peppers
or courgettes instead of leeks. If you prefer, bake the frittata in the oven for 20 to 25 minutes
at 190°C, fan oven 170°C, Gas Mark 5, until set and browned