To prepare vinaigrette: Puree cucumber, oil, vinegar, chives, parsley, yogurt, mustard, horseradish, sugar and salt in a blender until smooth.
To prepare salad: Season a wooden salad bowl by rubbing with garlic and pinch of salt. Chop the garlic and add to the bowl along with all the lettuce. Pour 1/8 cup of the vinaigrette over the greens; toss to coat. (Cover and refrigerate the remaining vinaigrette for up to 3 days.) Serve the salad garnished with radishes, scallions and hard-boiled egg.
Notes
Make Ahead Tip: Cover and refrigerate the vinaigrette for up to 3 days.