Follow the instructions found here to make the quick flaky pastry.
While the pastry chills, you can start with the filling: thinly slice the onions.
Take the pastry out and roll out the pastry to a 35.5 cm / 14-inch circle on a lightly floured surface. Then wrap it around your rolling pin and place it on a baking sheet.
Now, sprinkle on half the grated cheese over the pastry, leaving a 5 cm / 2-inch border all around the edge.
Over this, sprinkle the remainder of the cheese and bake for 28 minutes.
If the taste of Cheddar cheese is too strong for you, you could use Gruyère instead. You could also add a little grated nutmeg before you top the pastry with the caramelised onions.