SHORT CRUST PASTRY:
Preheat the oven to 350°F (180°C).
Place the flour and butter in a food processor. Whiz for a few seconds, then add half the beaten egg and continue blending. You might need to add a little more egg, but don’t add too much—the pastry should just come together. If making by hand, rub the butter into the flour, then use your hands to bring it together with the egg. Flatten out the ball of dough to a thickness of about 1 1/4 inches, wrap or cover with plastic, and place in the fridge for at least 30 minutes.
When you are ready to roll the pastry, remove from the fridge. Place the pastry between two sheets of plastic, which should be bigger than your tart pan. Using a rolling pin, roll it out until it is no thicker than 1/8 inch. If the pan is round, make sure to keep the dough round also, and large enough to line the base and sides of the pan.
Line a 10-inch ovenproof pie plate with the pastry. Trim it so that it is a bit bigger than the plate, and then fold up the edges slightly so that you have a slight lip all the way around. This will prevent the cream from running over. Place the pastry in the fridge while you prepare the filling ingredients.
Melt the butter in a small saucepan, add the onion and cook over a gentle heat until soft. Set aside. Whisk two of the eggs in a bowl, add the cream, the cooked onion, chopped ham and parsley. Season with salt and pepper to taste. Pour this into the pastry shell. Carefully break the remaining eggs onto the tart, trying to keep the egg yolks intact.
Bake for 25–35 minutes in the preheated oven until the custard is set in the center and the eggs on top are just cooked. Cut slices of the tart straight from the dish. Serve warm or allow to cool and pack for a picnic.