Pour vegetable oil into a large skillet or wok to a depth of about 1 inch and heat to 350°F (medium-high).
In a medium bowl, whisk 2 egg whites until frothy, then add the chicken pieces and toss to coat evenly.
In a separate bowl, combine 1/4 cup cornstarch, 1/4 cup all-purpose flour, 1/2 teaspoon salt, and 1/4 teaspoon black pepper.
Dredge the egg-coated chicken in the cornstarch-flour mixture, shaking off any excess coating.
Fry the chicken in batches until golden brown and crispy, about 3 to 4 minutes per batch; transfer to a paper towel–lined plate to drain.
Meanwhile, make the sauce: in a saucepan over medium heat combine 3/4 cup chicken broth, 1/3 cup lemon juice, 1/4 cup sugar, 2 tablespoons honey, 1 tablespoon lemon zest, and 1 tablespoon soy sauce; bring to a simmer.
Whisk 2 tablespoons cornstarch with 2 tablespoons water to form a smooth slurry, then stir it into the simmering sauce and cook until thickened, about 2 to 3 minutes.
Remove the sauce from heat and stir in 1 teaspoon sesame oil.
Toss the fried chicken pieces in the warm lemon sauce until evenly coated.
Garnish with lemon slices, thinly sliced green onions, and sesame seeds before serving.
