Preheat oven to 400°F. Line a baking sheet with parchment paper.
Spread ½ cup shredded cheddar in a circle (about 5 inches wide) on the parchment. Lay 2 pickle planks side by side on top. Sprinkle another ½ cup cheese over the pickles. Repeat for 3 more "buns" (total 4 pieces).
Bake 16 minutes until cheese is golden and crispy around edges. Watch closely after 12 minutes to avoid burning.
Let cool on baking sheet 10 minutes-this makes the cheese crisp and sturdy.
Carefully lift one "bun." Spread mayo on it. Layer turkey, cheese, lettuce, tomato, and onion. Top with another "bun." Repeat for second sandwich.
Slice in half if desired. Eat right away for max crunch.