1. Combine ricotta cheese and yogurt.
2. Line a colander with a coffee filter or several layers of clean, dry cheesecloth. Place cheese-yogurt mixture in colander and set over a deep bowl. Cover with plastic wrap and refrigerate for 8 hours. Discard liquid in bowl.
3. Combine yogurt-cheese mixture with scallion, lemon peel, and pepper.
4. Stuff each celery stalk with 1 tablespoon of yogurt-cheese mixture.
Variation Delete scallions and lemon peel. Add 1 tablespoon grated carrot, 1 tablespoon minced green bell pepper, 1 tablespoon finely chopped cucumber, and 1 tablespoon fresh minced parsley. Serve in hollowed-out cherry tomatoes instead of celery sticks.