Combine syrup, egg whites, and iced tea in large pitcher, bain-marie or another container. Whisk until fully incorporated.
Fill cups three-quarters full of ice. Pour iced tea mixture over ice halfway up.
Whisk remaining tea mixture vigorously until a thick a creamy foam gets created or use an electric stick blender to quickly foam. Divide this mixture into glasses.
Garnish with thin lemon slice, and/or mint.
Combine syrup, egg whites and iced tea with whisk until combined well.
Transfer to sterilized bottles, leaving about ½-inch headspace inside bottle for foam to fill.
Cap and shake bottles vigorously for 10 seconds.
Keep refrigerated until consumed.