Prepare the vinaigrette by whisking together sherry vinegar, walnut oil, peanut oil, finely chopped chervil, salt, and freshly ground pepper until emulsified.
Peel and wash the sweet potato in several changes of water, drain, and spin dry.
As an experienced chef, I recommend cooking the thornback ray in court-bouillon or salted water, timing based on thickness—10 minutes for thin fins, up to 25 for thicker ones. Remove the skin from both sides and gently lift the flesh to keep it intact.
Toss the lamb's lettuce with half the vinaigrette and arrange on plates as a fresh bed.
Season the ray lightly and place it atop the salad. Sprinkle with remaining chopped chervil.
Note: Always check doneness by ensuring the flesh pulls easily from the bone for perfectly tender results.
