Sauce: stir mustard, vinegar and oil well, mix in onion and parsley, season sauce.
Lettuce: cut cooked potatoes into halves or quarters according to size. Heat clarified butter in a frying pan, roast potatoes over medium heat, turning occasionally, for about fifteen minutes until golden brown, season with salt, remove, cool slightly. Dab remaining drippings with paper towel, set pan aside.
Halve cucumber lengthwise, remove seeds, cut into 5 mm thick slices, quarter radishes.
Mix potatoes, cucumber and radish with the sauce.
Shrimps: rinse when cooled, rub dry. Mix the juice of one lemon with the remaining ingredients and coat the shrimps with the mixture. Cover and marinate in refrigerator for about thirty minutes, rub dry.
Heat clarified butter in a frying pan set aside. Fry shrimp in batches on both sides for about THREE min each, season with salt.
Serve: Arrange potato salad on a large platter, spread shrimp evenly on top.
Tip: Instead of clarified butter, you can also use butter in most cases.