Combine water and buckwheat in a saucepan; bring to a boil. Reduce heat; simmer until water is absorbed, about 10 minutes.
Pour lime juice into a bowl; add onion, tomato, Kalamata olives, mint, and dill. Stir in olive oil, rice-wine vinegar, cumin, salt, and pepper. Add buckwheat; stir until ingredients are combined. Top with feta cheese.