Roll out the pastry into an even circle a bit larger than your flan tin.
Place on top, and relax into the tin, making sure it is pressed into the corners. Cut off any overhanging extra pastry with a sharp knife. Lay some baking paper in the pastry, and add baking beans. Blind bake for 15 minutes, remove the beans and paper, and bake for a further 10 minutes. Leave to cool.
Finely dice the onion, and dry fry (no oil) with the bacon over a medium heat until the onions are soft and the bacon is cooked.
Meanwhile, whisk the eggs, milk, cream, salt and pepper together. Grate the cheese into a bowl.
Spread the bacon and onion evenly in the pastry case, followed by the grated cheese, and then pour the egg mixture over the top. If you want, you can mix it up, but this is not needed.
Bake for 30-40 minutes until the top is golden brown.
Notes
Alternatively, use bacon rashers, and simply cut them up into smaller pieces.The ratio of milk and cream can be changed if you fancy a creamier quiche, but ideally don't use any less cream than is written.