1. Bring a large saucepan or Dutch oven of water to a boil. Place peas in a mesh strainer; set in boiling water for 1 minute. Rinse peas under cold water. Place 1 cup peas in a blender or food processor; reserve remaining peas.
2. Cook pasta in the boiling water according to package directions, omitting salt and fat; drain. Rinse under cold water until cool; drain.
3. Add 3 tablespoons water, 1 tablespoon tarragon, oil, rind, salt, pepper, and garlic to peas in blender. Process until smooth. Combine pea mixture and pasta in a large bowl; toss to coat. Add reserved peas, remaining 1 tablespoon tarragon, watercress, and juice; toss gently to combine. Sprinkle with feta; serve immediately.