Preheat the oven to 200°C.
Roll out the pastry on a floured surface to line a 20cm tart tin.
Chill the pastry-lined tin in the fridge for 30 minutes.
Line the pastry with greaseproof paper, fill with baking beans, and bake for 15 minutes.
Remove the weights and paper, then bake for another 5-6 minutes until golden.
Reduce the oven temperature to 180°C.
Melt butter in a pan and fry leeks until softened, then let them cool.
Whisk the cream, egg, and egg yolk together in a bowl.
Combine the egg mixture with the cooled leeks and season with salt and pepper.
Pour the leek mixture into the baked pastry case.
Layer the smoked salmon over the leek filling to cover it completely.
Sprinkle grated gruyère over the salmon and bake for 20 minutes.
Whisk mustard, vinegar, and olive oil to create a dressing.
Toss the lettuce and watercress with the dressing.
Serve a slice of tart alongside a portion of salad.