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In a large bowl, mix together red pepper, carrot, sweetcorn, sugar snap peas, beetroot and tuna.
In a small bowl, mix together mayonnaise, lemon juice, wholegrain mustard, water and seasoning, if you like, to make a dressing.
Line a large bowl with lettuce leaves, fill with the salad and drizzle with dressing. Top with spring onion and serve straight away.