Caramelised onion and walnut soufflé tart

Method

1. Heat the oil and half the 50g butter in a large frying pan, then add the sliced onions and the thyme leaves. Season well, cover, then cook on a very low heat for 45 minutes. Uncover and cook for a further 15 minutes or until the onions are a rich golden brown. Set aside.
2. Meanwhile, make the pastry. Put the 250g flour, walnuts and salt in a food processor and pulse to combine. Add the butter and pulse until the mixture resembles breadcrumbs.
3. Add the egg yolk and pulse, gradually adding the water until the pastry just comes together. Tip out onto a lightly floured surface, bring together with your hands and knead briefly. Shape into a disc, wrap in cling film, then chill for 15 minutes.
4. Heat the oven to 200°C/fan180°C/gas 6. Roll out the pastry on a lightly floured surface to the thickness of a pound coin, then use to line a 23cm loose-bottomed fluted tart tin (see tip). Chill for 10 minutes.
5. Line the pastry case with baking paper and baking beans or rice, then blind bake for 15 minutes. Remove the paper and beans/rice, then return to the oven and bake for 5-10 minutes more or until golden. Remove from the oven. Turn the oven down to 180°C/fan160°C/gas 4.
6. Heat the remaining butter in a pan and, when foaming, stir in the 50g flour and cook for 2 minutes. Gradually add the milk, stirring, until you have a thick white sauce. Season well, then bubble for a minute or two. Remove from the heat and stir in the cream, cheeses and mustard, then taste and adjust the seasoning. Pour into a big bowl and stir in the egg yolks and chives.
7. In a clean mixing bowl, whisk the 3 egg whites to soft peaks, then fold them into the egg mixture, being careful not to knock out all the air.
8. Spread the base of the tart case with the onions, then pour over the soufflé mixture. Transfer to the oven and bake for 30 minutes or until set and puffy. Serve immediately.