Roll the pastry out thinly on a lightly floured surface, then stamp out 12 x 10 cm (4 inch) circles, with a plain biscuit cutter, and press into a buttered 12-section muffin tin. Reknead and reroll the pastry trimmings as needed. Chill for 15 minutes.
Heat the butter in a frying pan, add the onions and fry over a gentle heat for 10 minutes, stirring from time to time until softened and just beginning to colour.
Add the eggs, milk and mustard to a large wide-necked jug, and fork together until just mixed. Add the cheese and seasoning and mix together. Divide the mixture between the pastry cases, then spoon in the fried onions.
Bake in a preheated oven, 190°C (375°F), Gas Mark 5, for 20–25 minutes until golden brown and the filling is just set. Leave to cool for 10 minutes, then loosen the edges of the tarts with a knife and remove from the tins. Serve warm or cold with salad.