Place chicken breasts in a zip-top bag and pour in the pickle juice. Press out air and seal. Refrigerate for 30 minutes up to 4 hours. Longer = tangier and more tender. Don't go overnight—it can get too salty.
Heat oven to 425°F (220°C). Place a wire rack on a rimmed sheet pan and lightly spray the rack. This is your secret to crispy edges.
In a small bowl, combine Dijon and mayo. Pat chicken dry, then brush both sides lightly with the mixture. This helps the crumbs cling like a champ.
In a shallow dish, stir together panko, whole-wheat crumbs, paprika, garlic powder, onion powder, cayenne, salt, pepper, and Parmesan. Drizzle with 1 tablespoon olive oil and toss to distribute (or coat with cooking spray). Even oiling = even browning.
Press each chicken breast firmly into the crumb mixture, coating both sides. Really press—no timid breading here.
Place on the prepared rack. Lightly spray the tops with oil or brush with remaining olive oil. Bake 15–20 minutes, flipping at minute 12 if you want extra even color. Target internal temp is 165°F (74°C) .
While the chicken finishes, toast the whole-wheat buns. Stir together the optional sauce and slice your toppings.
Spread sauce on buns. Add lettuce, chicken, pickles, tomato, onion, and a squeeze of lemon if you're fancy. Top with bun and let it rest 1 minute to settle.
Slice in half (diagonal for the drama) and serve hot. Expect vanishing acts.