Cut the pastry into 6, then roll one portion out between 2 sheets of clingfilm until a little larger than a buttered individual 10 cm (4 inch) loose-bottomed fluted tart tin. Remove the top sheet of clingfilm, turn the pastry over, drape into the tart tin and remove the top sheet of clingfilm. Press the pastry into the base and sides of the tin with fingers dusted in rice flour. Trim off the excess pastry with scissors a little above the top of the tin. Patch any cracks or breaks with pastry trimmings. Repeat until 6 tarts have been made. Put on a baking sheet and chill for 15 minutes.
Meanwhile, to make the filling, heat the oil in a frying pan, add the bacon and onion and fry for 5 minutes, stirring until golden.
Divide three-quarters of the cheese between the tart cases, then sprinkle the onion and bacon mix on top. Beat the eggs, milk and mustard in a jug with a little salt and pepper, then pour into the tarts. Sprinkle with the chives, if using, and the remaining cheese.
Bake in a preheated oven, 190°C (375°F), Gas Mark 5, for 25–30 minutes until the tops are golden and the pastry bases cooked through. Leave to cool for 5 minutes, then remove from the tins and serve with salad.