1. To make the pastry, put the flour into a bowl and mix in the salt. Add the butter and lard and rub into the flour with your fingertips until the mixture resembles fine breadcrumbs. Alternatively, do this in a food processor or mixer and then transfer to a bowl.
2. Now work in just enough cold water to bring the dough together. When it begins to stick together, gently knead it into a ball. Wrap in cling film and chill in the fridge for around 30 minutes.
3. Meanwhile, for the filling, thinly slice the shallots and onions. Heat the butter and oil in a large frying pan over a medium-low heat. Add the shallots and onions with a pinch of salt and cook slowly for at least 20 minutes, stirring occasionally, until they are very soft and golden. Season with pepper and more salt if needed. Leave to cool.
4. Heat your oven to 200°C/gas 6 and have ready a 23cm loose-based fluted tart tin, 3.5cm deep.
5. Roll out the pastry on a lightly floured surface to a 3mm thickness and use it to line the tart tin, leaving the excess hanging over the edge. Keep a little uncooked pastry back in case you need to patch any cracks later. Prick the pastry base with a fork. Line the pastry with baking parchment or foil and then fill with baking beans, or uncooked rice or lentils.
6. Bake blind for 15 minutes, then remove the parchment and baking beans and return the pastry to the oven for about 8 minutes or until it looks dry and faintly coloured. Trim away the excess pastry from the edge. Use a tiny bit of the reserved raw pastry to patch cracks or holes if necessary. Turn the oven down to 180°C/gas 4.
7. For the filing, whisk the eggs, egg yolks and cream together, then whisk in the mustard and chives. Season with salt and white pepper.
8. Spoon the cooled onion mixture into the pastry case and spread it evenly, then carefully pour on the egg mixture. Bake in the oven for 30-35 minutes, until the filling is just set and golden. Leave in the tin for 5 minutes, then unmould. Serve warm or cold..