In a large skillet or wok, heat vegetable oil to 350°F (about 1 inch deep).
In a medium bowl, whisk egg whites until frothy. Add chicken pieces and toss to coat.
In a separate bowl, mix cornstarch, all-purpose flour, fine sea salt, and black pepper.
Dredge the egg-coated chicken in the cornstarch mixture, shaking off excess.
Fry chicken in batches until golden brown and crispy, about 3 to 4 minutes per batch. Transfer to a paper towel-lined plate.
For the sauce, combine chicken broth, fresh lemon juice, granulated sugar, honey, lemon zest, and soy sauce in a saucepan over medium heat.
In a separate bowl, stir together cornstarch and water to make a slurry and set aside.
Bring sauce to a simmer, then stir in the slurry. Cook until sauce thickens, about 2-3 minutes.
Remove from heat and stir in sesame oil.
Toss fried chicken pieces in the warm lemon sauce until well coated.
Garnish with lemon slices, green onions, and sesame seeds before serving, if desired.