Method
1. For the pastry, sift the flour, mustard and a pinch of salt into a food processor. Add the butter and lard and whizz into fine crumbs. Add the cheese and 1-2 tbsp cold water and pulse until it forms a ball. Turn out and knead briefly until smooth.
2. Lightly butter a 26cm loose-bottomed tart tin that’s 4cm deep. Thinly roll the pastry out on a floured surface and use to line the tin. Prick the base all over with a fork and chill for 20 minutes. Preheat the oven to 200°C/fan180°C/gas 6. Line the pastry case with baking paper and baking beans or rice, bake for 20 minutes, then remove the paper and beans. Bake for 5 minutes more until golden, then remove and lower the temperature to 180°C/fan160°C/gas 4.
3. For the filling, coarsely grate the courgettes, toss them with the salt and drain in a colander for 10 minutes. Squeeze out the excess liquid and dry on kitchen paper. Melt half the butter in a pan, add the flour and cook for 1 minute. Remove from the heat and gradually stir in the milk. Return to the heat and bring to the boil, stirring continuously, until thick and smooth.
4. Meanwhile, in a large frying pan, fry the courgettes in the remaining butter for 2 minutes until tender. Stir into the sauce with both cheeses and the mint. Season to taste. Cool slightly and mix in the egg yolks and cream.
5. In a clean bowl, whisk the egg whites to soft peaks, then fold into the courgette mixture with a large metal spoon. Pour into the pastry case and bake for 35-40 minutes until golden brown. Cool in the tin for 5 minutes, and serve warm.