To make the pastry case, sift the ﬂour, salt and mustard into a bowl and rub in the butter with your fingers until the mixture resembles ﬁne breadcrumbs. Mix in the cheese and then gradually mix in the egg yolk and water to form a stiff dough. Knead the dough on a lightly ﬂoured surface until smooth, then roll it out and line a 23 cm ﬂan tin. Trim the edge and prick well all over the base with a fork. Refrigerate for 30 minutes.
Meanwhile, heat the oven to 220°C. Line the pastry case with crumpled greaseproof paper, ﬁll with baking beans and bake blind for 10 minutes. Remove the paper and baking beans and bake in the centre of the oven for 10–15 minutes more, or until lightly browned.
When the pastry case has been cooking for about 15 minutes, start to make the ﬁlling. Melt the butter in a saucepan, stir in the ﬂour and cook over low heat for 1 minute. Gradually stir in the milk and bring to the boil, stirring continuously until the sauce becomes very thick. Add the ﬁnely grated Cheddar cheese and stir over medium heat until the cheese melts and the mixture becomes smooth, but take care not to overheat. Remove the pan from the heat, beat in the egg yolks, one at a time, then stir in the cream. Season well with pepper.
Remove the pastry case from the oven and ﬁll it with the cheese sauce. Whisk the egg whites for the topping with salt and cayenne pepper until very stiff. Quickly spoon the mixture over the cheese ﬁlling to cover it completely. Sprinkle the top ﬁrst with cheese, then with breadcrumbs, and return the flan to the oven. Bake for 15 minutes, or until golden brown on top. Serve hot, with a green salad.