1. Preheat oven to 400 degrees F.
2. With a vegetable peeler, peel the outer skin of lemon into strips. Cut into thin strips and chop finely. Cut lemon in half and squeeze half for juice. Set aside.
3. In a medium saucepan, on medium setting, heat milk, half the sugar and scrapings from ½ the vanilla bean and lemon zest. Stir. Discard the pod or place in sugar bowl.
4. Meanwhile, in a medium bowl, whisk together the remaining sugar, flour and egg yolks until blended and pale yellow.
5. Once milk has just come to the boil remove from heat and pour ¼ cup of hot milk into the egg mixture to temper, while whisking.
6. Return saucepan to heat and pour in the tempered egg mixture gradually while whisking over medium heat. Keep whisking for about 4 to 5 minutes until thick and boiling. Remove from heat and stir in lemon juice to blend. Cover immediately with plastic wrap, right on surface of custard, to prevent skin from forming. This step can be done in advance and kept in fridge.
7. Butter 6x 6 ounce ramekins and sprinkle well with sugar, tapping out excess. Fill a medium baking dish.
8. Whip 5 egg whites in a clean bowl, on medium speed until light and fluffy. Increase to high speed and continue to whip while gradually until stiff peaks hold, but are not grainy. Fold into cheese mixture and pour into 6-6 ounce ramekins, filling only 2/3 full.
9. Place ramekins in water bath. Bake for 20-25 minutes or until golden and puffed.