Coarsely chop 6 Persian cucumbers (1 lb. total) into 1/2-inch chunks; place in a serving bowl. With a rubber spatula, press 1 chopped hard-cooked egg through a wire sieve onto cucumbers (or dice egg finely). Whisk together 1 tablespoon Dijon mustard and 1 teaspoon lemon juice, whisk in 3 tablespoons extra-virgin olive oil until thick, add salt and pepper to taste, and drizzle over salad. Sprinkle with 1 tablespoon chopped fresh tarragon leaves.