Begin by horizontally slicing the boneless, skinless chicken breasts into thinner fillets, ensuring they are even in thickness.

In a shallow bowl, combine all-purpose flour, salt, black pepper, garlic powder, onion powder, paprika, and cayenne pepper. Whisk until fully blended.

In another shallow bowl, whisk together the large egg and water until smooth to create the egg wash.

Dredge each chicken fillet in the flour mixture, shake off excess, then dip in the egg wash, and finally roll in the breadcrumbs.

Heat about an inch of vegetable oil over medium-high heat until shimmering.

Carefully add the coated chicken fillets to the hot oil and fry for about 4-5 minutes on one side, until golden brown.

Flip the chicken and cook for another 4-5 minutes until fully cooked and crispy. Drain on paper towels.

Toast the hamburger buns in a skillet or under the broiler until golden-brown.

In a small bowl, mix together mayonnaise, Dijon mustard, lemon juice, and lemon zest until fully combined.

Spread the creamy sauce on the toasted sides of the buns, layer with crispy chicken, shredded lettuce, and tomato slices.

Serve the sandwich warm for the best taste.