Seville orange tart

Name: Seville orange tart
Prep time: 30 min
Cook time: 1 h 10 min
Total time: 1 h 40 min
Cuisine: British
Ingredients: 4 seville oranges (or the grated zest of 4 small oranges and the juice of 2 lemons and 2 oranges); 6 medium free-range eggs; 250g caster sugar; 150ml double cream; 175g plain flour, plus extra for dusting; 50g icing sugar; 100g chilled butter, cut into small pieces, plus extra for greasing; Finely grated zest of 1 small orange (not seville); 1 large free-range egg, separated; 2 tsp freshly squeezed orange juice