1 unwaxed lemon, zest finely pared
1 lime, zest finely pared
3 level tbsp sweetener
300g fat-free natural Greek-style yogurt
1 large egg white*
1 Put the lemon and lime zest in a small saucepan with 1 level tbsp of the sweetener and 150ml water. Bring to the boil, then reduce the heat to low and simmer very gently for 10-12 minutes, or until the zest is softened. Drain the zest in a sieve, reserving the cooking liquid. Set aside the zest for decoration.
2 Put the yogurt in a mixing bowl, and whisk in the reserved citrus liquid and the remaining sweetener.
3 Put the egg white in a clean glass bowl and beat with an electric hand whisk on a medium speed until it forms stiff peaks. Using a metal spoon, gently fold into the yogurt mixture. Divide between 4 dessert glasses and chill for 1-2 hours. Decorate with the reserved zest.