To make the pastry, rub the butter, flour and Parmesan together in a bowl until the mixture has the consistency of fine breadcrumbs. Add the egg mix and quickly bring it all together into a dough. Knead lightly a few times with floured hands until smooth. Shape into a flat ball, cover in cling film and chill for 30 minutes. When ready, roll out the pastry on a floured surface and use to line a 22cm deep flan tin, or 8 x 10cm small individual tins. Next, prepare the filling by heating the olive oil in a frying pan and fry the onion for 3 minutes until softened. Add the garlic and bacon to the pan and continue to cook for 2-3 minutes and allow to cool slightly. Meanwhile, whisk the eggs and cream with the chives and season well to taste. Divide the onion, bacon and cooked mussels into the large pastry case or individual cases and transfer onto a baking sheet. Pour in the egg and cream mixture and scatter over the Brie cheese cubes. Bake in the oven at 180`c for approximately 15 minutes until the filling is just firm and golden. If using a large flan tin the cooking time will vary to approximately 45 minutes. Serve the tart whilst still warm with the salad leaves.