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To make the tzatziki, add equal parts Greek yogurt and mayo to a small bowl. Grate the cucumber into the bowl and squeeze in one lemon wedge. Add chopped dill, a couple of dashes of garlic powder, salt, and pepper to taste.
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Turn on the oven broiler to high. While the oven warms, generously butter two pieces of bread (face side up) on a cookie sheet. Layer on enough slices of Parmesan to cover both faces of bread. Add garlic powder, salt, and freshly cracked pepper. Place the cookie sheet in the oven and broil for 3 mins or so. Make sure the bread does not burn.
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While the bread gets toasty, prepare all the sandwich fillings. When the bread is nicely toasted and the Parmesan is melted, remove from the oven and start building the sandwich.
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Layer on lettuce, cucumber, tomato, avocado, and warmed chicken slices. Top with tzatziki and Dijon mustard (optional).
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Cut in half and enjoy!