Preheat the oven to 200°C/180°C fan (400°F), Gas Mark 6. Unroll the pastry onto a large baking tray lined with baking parchment and lightly score a 1cm (½ inch) border all the way around. Beat the egg yolk and brush it over the border to glaze. Toss the prepared vegetables with the oil and spread out on a second tray. Place both in the oven, with the vegetables on the top shelf, for 20 minutes, until the pastry has puffed up and the veg is just tender. Press down the centre of the pastry with the back of a spoon.
In a bowl, mix the 3 eggs with the crème fraîche, goat’s cheese, mustard, the herbs and some salt and pepper. Spread over the tart base, avoiding the border. Arrange the veg on top then return to the oven for about 20 minutes until the filling has set.
Make a dressing by whisking together the oil, mustard, vinegar and a little salt and pepper. Remove the tart from the oven and cool a little before scattering over the watercress and drizzling with the dressing.