Rinse the lettuce, shake dry and tear into small pieces. Drain the tuna well and in a bowl, pick apart with a fork. Trim, rinse and chop the spring onions. Rinse the basil, shake dry, pluck off the leaves and cut into fine strips. Mix the spring onions with basil and mayonnaise and stir into the tuna. Season with salt, lemon juice and pepper and spread on 2 pieces of bread. Top with the lettuce and cover with the remaining 2 slices of bread. Use light pressure to cut sandwich into triangles.
Place the sandwiches as desired on a plate and serve.