Grease a 26cm tart tin and line with with shrotcrust pastry sheet. Fill with baking beans (you can use a smaller tin or dried pulses such as lentils or rice if you do not have baking beads) and blind bake for 20 minutes. Remove from oven and turn the temperature down to 160oC / Fan 140oC / gas mark 3).
Heat the oil in pan and cook onions until soft, then place into the pastry case along with the watercress and smoked salmon.
Whisk the eggs with the mascarpone, 75g of the cheddar cheese and the chives. Pour into the pastry mix and sprinkle the remaining cheese on top.
Bake for 30-40 minutes until golden brown.
Can be served warm, at room temperature or once chilled.