For the quick shortcrust pastry, put the flour, salt, butter and lard into the bowl of a food processor. Pulse until the mixture looks like fine breadcrumbs. With the processor running, slowly add 3-4 tablespoons of cold water processing just until the dough comes together in clumps. You may need a little less water or a touch more. Turn out the dough onto a lightly floured surface and knead briefly until smooth. Wrap in cling film and leave in the fridge to chill for around 30 minutes.
Meanwhile, for the filling, melt the butter and oil in a large frying pan over a medium-low heat. Add the shallots and onions and a pinch of salt. Cook slowly for at least 20 minutes, stirring from time to time, until the mixture is very soft and golden-brown.
Season with pepper and more salt if needed, then leave to cool.
Heat your oven to 200C/400F/Gas 6 and have ready a 23cm/9in fluted loose-bottomed tart tin, 3.5cm/1¼in deep.
Roll out the pastry on a lightly floured surface to a 3mm thickness and use it to line the tart tin, leaving excess pastry hanging over the edge. Keep a little uncooked pastry back in case you need to patch any cracks later. Line the pastry with baking parchment or foil and then fill with baking beans, or uncooked rice or lentils.
Bake blind for 15 minutes, then remove the parchment and baking beans and return the pastry to the oven for about eight minutes or until it looks dry and faintly coloured.
Use a small, sharp knife to trim away the excess pastry from the edge. Use a tiny bit of the reserved raw pastry to patch any cracks or holes if necessary.
For the filling, whisk the eggs, egg yolks, cream, mustard and chives together. Season with salt and a pinch of white pepper. Add the cooled onion mixture and stir to combine.
Pour the mixture into the pastry case and cook for 25-30 minutes, or until the filling is just set and golden-brown. Leave to cool in the tin for five minutes before removing, then serve warm or cold.