Place chicken between 2 sheets of heavy plastic on a solid, level surface. Firmly pound chicken with the smooth side of a meat mallet to an even thickness.
Whisk egg and water together in a bowl.
Mix panko bread crumbs, lemon pepper, and dill weed together on a flat dish.
Dip chicken pieces in egg mixture. Dredge in panko mixture until coated.
Coat a large skillet with cooking spray; preheat over medium-high heat. Cook chicken pieces in batches until golden brown, about 5 minutes per side. Drain on paper towels. Sprinkle lemon juice over each piece of chicken.