Full preparation
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Stir together the tuna, apples, bell pepper, dill, parsley, walnuts, lemon juice, mayonnaise and mustard in a medium bowl. Set aside.
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To assemble the sandwich, for the first layer, spread a desirable amount of the tuna mixture and top with the avocado slices.
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For the second layer, spread some mayonnaise and top with the lettuce, cucumber and a slice of bread.