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Drain the tuna.
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Mix the tuna, 1/4 cup mayo and 1/2 tsp. mustard together in a large bowl until smooth.
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Thinly slice cabbage then chop it up into small pieces.
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Roll up the spinach leaves tightly and slice thinly into ribbons.
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Chop the green apple into small pieces.
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Add the chopped cabbage, spinach and apple to the bowl with the tuna and mix.
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Add the 1/2 tsp. salt, 1/2 tsp. garlic powder and 1 tsp. apple cider vinegar.
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Mix everything thoroughly, squeezing 1/2 a lemon over the top.