Place potatoes in a large saucepan; add water to cover. Bring to a boil. Reduce heat; cook, uncovered, until tender, 8-10 minutes. Meanwhile, chop green onions, reserving white portions for salad. Add green portions to a blender. Add avocados, parsley, mayonnaise, tarragon, capers and seasoned salt. Cover and process until blended, scraping down sides as needed. Drain potatoes; transfer to a large bowl. Add celery, white portions of green onions, and dressing; toss to coat. Refrigerate, covered, at least 1 hour. Top with radishes and additional parsley.