1 tbsp oil
4 large Spanish onions, sliced
1 garlic clove, crushed
2 medium eggs and 2 medium egg yolks
50g wholegrain mustard
142ml carton double cream
2 tbsp chopped fresh mint leaves
For the pastry
225g plain flour, plus extra for dusting
100g butter, cubed
25g caster sugar
1 medium egg, beaten
1. Pulse all the pastry ingredients together in a processor – add a dash of water so it comes together. Shape into a ball, wrap in cling film and chill.
2. Meanwhile, melt the butter and oil together in a large frying pan over a medium-low heat. Add the onions and garlic. Season and cook for 30 minutes, stirring often, until golden. Set aside.
3. Preheat the oven to 200°C/fan180°C/gas 6. Roll out the pastry and use to line a 23cm x 3cm deep fluted, loose-bottomed tart tin. Fill with baking paper and beans, and bake for 15 minutes. Remove the paper and beans, and bake for a further 10 minutes, until golden.
4. Lower the oven temperature to 180°C/fan160°C/gas 4. In a bowl, whisk together the eggs, yolks, mustard and cream. Stir in the onions and mint. Season, spoon into the tart case, and bake for 20 minutes, until just set. Cool slightly, then remove and serve.