Firstly, pre-heat oven to 200C​ and prepare by peeling and slicing onions to set aside​. To make the savoury custard filling, combine egg, liquid cream, thyme, salt and black pepper in a bowl to a smooth consistency, set aside.​ Place rolled out pastry dough into a tart pan and cut off any extra on the edges. Poke into the pastry with a fork to create small holes. (this is to avoid the pastry deforming when baking blind). ​ Place a cut out round baking sheet, which is fitting into the tart base, and keep beans or pie weights on the round cut out baking sheet. ​ Bake the crust alone (called baking blind) for 15 minutes at 200C. ​ Heat up a pan with the butter and sauté onions soft. This can take 10-15 minutes​. Place cooked onions into the quiche crust and spread out evenly.​ Pour savory custard mixture over the onions and bake quiche for 30 minutes at 200-220C. Serve and enjoy! ​