Preheat the griller to high. Trim the zucchini then shred coarsely in a food processor or grate by hand. Put into a colander with a little salt, toss gently together and put aside to drain thoroughly.
Rinse and dry the fish and make three diagonal cuts through the white skin on each one.
Put 4 sprigs dill aside for a garnish, then chop enough dill, chives and parsley to give 1 tablespoon of each. Put into a small bowl with half the butter, season well with pepper, and mash with a fork until well mixed. Divide the mixture into four and put aside.
Melt the remaining butter in a frying pan. Fry the zucchini over moderate heat for 10 minutes, until cooked but still slightly firm. Shake the pan gently from time to time.
Meanwhile, lightly oil a large griller tray and place the sole in it, white skin side up. Grill the fish for 5 minutes, then turn and sprinkle with the juice from the half lemon. Cook for 2–3 minutes, until the flesh flakes easily.
Cut the whole lemon into eight wedges and put aside. Place a portion of herb butter on each sole and warm under the griller for a few seconds until butter starts to melt.
Serve the fish on individual plates, accompanied by the zucchini and the lemon wedges.