Marinade the scored squid in horseradish and lemon juice for about ½ an hour.
Finely slice the celery and spring onions, chop the tomatoes in half, shave the cucumber and julienne the red capsicum.
Grill the drained squid on a hot BBQ until it curls up.
Toss the salad leaves with all the cut vegetables and the watercress and top with the grilled squid.
Mix together the mustard and olive oil into a dressing and pour over the salad.