Heat vegetable oil in a large skillet or wok to 350°F, about 1 inch deep.
Whisk egg whites in a medium bowl until frothy, then add chicken pieces and toss to coat evenly.
In another bowl, combine cornstarch, flour, salt, and pepper.
Dredge the egg-coated chicken in the flour mixture, shaking off excess.
Fry the chicken in batches until golden brown and crispy, about 3 to 4 minutes per batch, then transfer to a paper towel-lined plate.
In a saucepan over medium heat, combine chicken broth, lemon juice, sugar, honey, lemon zest, and soy sauce.
In a small bowl, whisk cornstarch and water to make a slurry, then stir into the simmering sauce.
Cook the sauce until it thickens, about 2 to 3 minutes, then remove from heat and stir in sesame oil.
Toss the fried chicken pieces in the warm lemon sauce until well coated.
Garnish with lemon slices, green onions, and sesame seeds if desired before serving.