For the tart, preheat the oven to 200C/400F/Gas 6.
On a lightly floured surface, roll out the pastry to line a 20cm/8in diameter, 2.5cm/1in deep loose-bottomed tart tin. Line the inside of the tin with the pastry, then cover and chill in the fridge for 30 minutes.
Remove the tart case from the fridge. Line the pastry tart case with greaseproof paper and fill with baking beans or uncooked rice. Bake blind in the oven for 15 minutes.
Remove the baking beans or rice and greaseproof paper and return the tart case to the oven to bake for a further 5-6 minutes, or until it is just cooked and becoming golden-brown.
Reduce the oven temperature to 180C/360F/Gas 4.
Meanwhile, melt the butter in a large frying pan over a low heat. Add the leeks and fry until just softened. Remove from the heat and leave to cool.
Place the cream, egg and egg yolk into a bowl and whisk to combine.
Add the egg and cream mixture to the cold leeks and mix together well. Season well with salt and freshly ground black pepper.
Pour the leek mixture into the cooked pastry case.
Place the smoked salmon over the top of the leek mixture filling, to cover the filling completely.
Sprinkle the grated gruyère over the top of the smoked salmon layer. Bake in the oven for 20 minutes, or until the filling is just set and the top is golden-brown.
For the salad, whisk the mustard, vinegar and olive oil together in a bowl. Add the Little Gems and watercress and mix well.
To serve, place a slice of tart onto each plate and place a portion of salad alongside.