Pat the scallops dry on kitchen paper and season lightly with salt and pepper. Finely chop or break the morcilla into small pieces.
Heat the oil in a small frying pan and gently fry the scallops for 1 minute on each side. Transfer them to the cleaned shells or to small, warm serving dishes if the scallops were ready-shelled.
Add the spring onion, lemon thyme and morcilla to the pan and heat gently, stirring, for 1 minute. Season lightly with salt and pepper and spoon over the scallops with the cooking juices to serve.