Start by making the pastry. Rub the butter into the flour until you have a crumb (a food processor or electric mixer + paddle attachment can help at this stage!) Then, add the water to bring the pastry together. Rest in a Tupperware in the fridge for about half an hour.
Roll your pastry out on a floured surface to about 3mm thick and line a 20cm tart tin. Using parchment and baking beans, blind bake at 170° for 25 minutes, then without the beans for a further 10 minutes or so until golden brown and crisp.
For the filling, blanch your nettles for one minute in boiling salted water. Drain and refresh in cold water. Then squeeze as much water out of them as possible (they will no longer be stinging!) and roughly chop them.
Beat together the cream and eggs, then stir through your nettles, herbs, the crumbled feta, half of the grated yarg and salt and pepper.
Pour the filling into your tart case, sprinkle with the remaining yarg and sesame seeds, then return to the oven for 30 minutes until golden brown and just set.
Enjoy with some new potatoes and a spring salad!