Scrub the potatoes (no need to peel them) and cut them into small chunks. Place them with the garlic clove in a steamer, sprinkle with salt, and steam for 10 minutes, or until the potatoes are just cooked through (the tip of a knife should meet no resistance).
Scrub the black radish (no need to peel it, unless it is old and its skin is really tough) and carve out any crack or hole that may harbor dirt. Grate using the large holes of a box grater. Go through the pile of grated radish and check for any brown bits, which may taste unpleasantly earthy. Place grated radish in a medium salad bowl, sprinkle with salt, toss to coat, and set aside while the potatoes are cooking; the salt will take the edge off the radish.
When the potatoes are cooked, set aside to cool until just slightly warm. Chop the steamed garlic clove finely and add it to the salad bowl. Add the vinegar, oil, and paprika, and toss to combine.
Add the potatoes and the fresh herbs to the bowl, sprinkle with pepper, and toss gently to combine. Taste and adjust the seasoning. Top with the crumbled walnuts, and serve. Leftovers keep well until the next day.