Preheat the oven to 200C degrees.
Roll out the pastry on a floured surface. It should be round in shape and wide enough to cover a 23cm cake dish. It should also be about 0.5cm thick. Place the pastry into the dish, make sure to push it into the corners and leave some hanging over the edge. Trim off the remaining pastry but leave a small overhang. Keep any remaining pastry for fixing cracks. Using your index finger and thumb pinch the pastry along the edge. Using a fork prick the base of the pastry a few times. Pop the pastry into the fridge for 10 minutes.
Remove the pastry from the fridge. Crunch up some parchment paper then place that over the pastry. Place ceramic baking beans over the paper then place the pastry in the top third of the oven and bake for 15 minutes. After 15 minutes take it out of the oven, remove the paper and beans. Check for any cracks then pop it back into the oven and bake again for 5 minutes. Remove from the oven a leave to cool a little.
Heat a tablespoon of olive oil in a pan and cook up the onions until they are soft about 10 minutes. Once they are cooked remove them from the pan and wipe it clean. Leave the onions in a bowl until you are ready to use them.
Now melt ¼ cup/40g of butter in a hot pan. Add the mushrooms and cook until they are golden brown.
Break 6 large Pace Farm eggs into a large mixing bowl, add ¾ of a cup of heavy cream, ½ tablespoon of thyme leaves, 1 minced clove of garlic, ¼ teaspoon of grated nutmeg, salt and pepper and whisk well.Stir in half a cup of cheese, then add the mushrooms to the bowl.
Reduce the oven temperature to 160C degrees. Place the dish on a baking tray. Spread ½ a cup of parmesan cheese over the base of the pastry. Top that with the cooked onions. Then carefully pour the egg and mushroom mixture to the pastry. Be careful not to pour any mixture over the edge of the pastry.
Place the tray with your quiche on it into the oven on the middle shelf and bake for 35 to 40 minutes. The quiche is done when it is golden brown on top and the filling is firm.
Remove from the oven and leave to cool slightly before serving.