Place the cucumber in a colander in the sink and sprinkle with the kosher salt. Let sit and drain for 30 minutes. Dry thoroughly with paper towels, then chop very finely.
Combine the finely chopped cucumber with the yogurt, garlic, lemon juice and dill. Refrigerate for at least 2 hours before serving. The sauce can be kept in the refrigerator in an airtight container for up to three days.