For the pastry, tip the flour and salt into a bowl and mix well. Rub the butter and lard into the flour until it resembles fine breadcrumbs. Add 3-5 tbsp ice-cold water and mix to firm dough.
Knead the dough briefly and gently on a floured surface. Wrap in cling film and chill for 30 minutes.
Roll out the pastry on a lightly floured surface to about 3mm thick and use to line a 25cm loose bottoms, round tart tin, 3.5cm deep, leaving a little excess pastry hanging over the edge. Prick the base all over with a fork, then chill for 15 minutes.
Heat the oven to 200C/fan 180C/gas 6. Line the pastry case with a sheet of greaseproof paper and fill with baking beans. Bake blind for 12-15 minutes, then remove the parchment and baking beans and return the pastry case to the oven for 5 minutes until the base is dry and the pastry is lightly covered. Remove from the oven and use a small, sharp knife to trim away the excess pastry from the edge.
For the filling, reduce the oven temperature to 180C/fan 160C/gas 4. Cook the pancetta in a frying pan over a medium heat until golden and the edges begin to crisp. Remove from the pan and then add the onion and cook in the pancetta fat over a low heat, until softened.
Beat the eggs, egg yolks and double cream together. Add the grated Parmesan, salt (not too much as the pancetta and Parmesan are salty) and pepper.
Scatter the pancetta and onion over the base of the pastry case. Spoon heaped teaspoons of the ricotta randomly in the pastry case, then scatter peas and lemon zest over the top.
Carefully pour the egg mixture into the tart case. Bake in the oven for 35-45 minutes, until the filling is just set and golden. Remove from the oven and leave to cool in the tin for 5 m inutes. Remove from the tin adn serve warm or cold.