In a large dish, mix the beaten eggs with the milk, grapefruit zest and cinnamon. Dip in the slices of challah until they’re fully coated, then leave to soak for about 30 mins (or covered overnight in the fridge).
Pour the grapefruit juice into a small saucepan with 1 tbsp sugar and set over a gentle heat until it dissolves. Turn up the heat and simmer vigorously until the sauce reduces and thickens. Set aside.
Tip the remaining sugar onto a large plate and stir with the mixed spice. Heat a large frying pan over a medium heat and add the butter. Once melted, carefully (they will be delicate) take 2 challah slices out of the egg mixture and put in the pan. Fry for 2-3 mins each side until browned. Meanwhile, reheat the grapefruit syrup over a gentle heat until warm. Fry the remaining challah slices. Dip each slice into the spiced sugar and turn until completely coated.
Serve with the grapefruit segments, a little of the grapefruit syrup and a dollop of crème fraîche.