Cut lemons into eight. Combine with 8 cups water in a large saucepan and bring to a boil. Boil for 5 minutes. Drain. Bring 3 cups of water to a boil in a small saucepan with sugar and salt, stirring to dissolve. Add lemon, cover and simmer over low heat until very tender (about 1 hour). Cool
Remove lemon from syrup. Remove and discard flesh reserving lemon peel. Cut peel into small dice (you will need ½ cup).
Bring stock to a boil. Stir in couscous, chopped lemon, oil, salt and pepper. Return to a boil. Remove from heat, cover and let stand for 10 minutes. Fluff with a fork and spread on a parchment lined pan to cool.
Transfer couscous to a large bowl and fluff with fork. Add parsley, lemon juice and toss.