Heat the oil in a frypan. Add onion and cook over low medium heat for 2- 3 minutes or until onion is soft golden brown. Set aside to cool.
Line the base and sides your tart pan with puff pastry, overlapping pastry where needed, pressing gently to form a good seal and ensuring pastry goes to edge of pan. Trim excess pastry from edges. Prick the base all over with a fork.
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 8-10 minutes or until it looks dry and faintly coloured. Remove from oven and allow to stand 10 minutes. Reduce oven temperature to 180C.
For the filling, whisk the eggs, egg yolks, cream, mustard and chives together. Season with salt and a pinch of pepper. Add Olssons salt, if using and 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.