Line a lightly buttered 10 inch tart tin with the pastry. Chill for 15 minutes.
Preheat the oven to 190ºC/375ºF/gas mark 5. Heat a baking tray in the oven until hot, then pop the tart tin onto the baking tray and bake blind for 10 mins. Remove the baking beans and paper and cook for another 5 mins, then trim the edges off the pastry.
Beat the eggs and then add the double cream, stir in most of the cheese and season the mixture.
Spread the Caramelised Red Onion Relish evenly over the base of the tart and pour the egg, cream and cheese mixture over the top. Sprinkle the top with the leftover cheese.
Bake in the middle of the oven for 20 mins or until set and a light golden brown on the top. Serve with a big green salad.