In a medium saucepan add the water and 1 teaspoon kosher salt. Bring the water to a boil over high heat. Add the green beans and cook until they are tender crisp about 3-4 minutes. Drain the beans and rinse with cold water. Set aside.
In a small bowl, combine the lemon juice, mustard, freshly ground black pepper, and a pinch of kosher salt. Whisk to mix well. Gradually add the olive oil, whisking the mixture to fully incorporate.
In a medium bowl, combine the green beans, tuna, olives, bell pepper, red onion, and tarragon. Pour the vinaigrette over the tuna mixture and gently fold the ingredients to mix and coat the salad.
To assemble the wraps: Warm the wraps in a microwave on high heat for 10-15 seconds to prevent them from tearing. Sprinkle 1/4 of the feta cheese over the surface of one side of a tortilla. Press the feta gently into the tortilla. Spoon 1/4 of the filling on the bottom half of a tortilla in a 2-by-5-inch rectangle. Fold the right and left edges of the tortilla over the filling toward the center. Fold the bottom edge of the tortilla toward the center and gently roll until the tortilla is wrapped around the filling. Wrap in aluminum foil for your picnic. Repeat with the remaining tortillas