For the chicken:
1/4 cup cornstarch
1 teaspoon kosher salt
1/2 teaspoon freshly ground black pepper
1 1/2 pounds boneless, skinless chicken breasts or thighs, cut into 1 1/2-inch pieces
2 tablespoons vegetable oil
For the orange sauce:
1 tablespoon finely grated orange zest (from 1 orange)
3/4 cup freshly squeezed orange juice (from 2 oranges)
1/4 cup granulated sugar
3 tablespoons honey
1 tablespoon cornstarch
1 tablespoon low-sodium tamari or soy sauce
1 tablespoon rice vinegar
2 cloves garlic, minced
1 teaspoon toasted (Asian) sesame oil
1/2 teaspoon grated peeled fresh ginger
1/4 teaspoon red pepper flakes
Make the chicken: Mix the cornstarch, salt, and pepper together in a gallon-sized zip-top bag. Add the chicken pieces, seal the bag, and shake until the chicken is well-coated.
Heat the oil in a large nonstick skillet over medium-high heat until shimmering, about 3 minutes. Add the chicken in batches, making sure not to crowd the pan, and cook about 2 minutes per side. The chicken might not brown, but the cornstarch coating will become crispy and dry. The chicken will not be completely cooked at this point, and that is OK. Transfer the chicken to a 4-quart slow cooker and repeat with any remaining chicken.
Make the sauce: Place all the ingredients in a medium bowl and whisk to combine. Pour over the chicken.
Cover and cook until chicken reaches 165°F on an instant-read thermometer, 1 1/2 to 2 hours on the LOW setting or 45 minutes to 1 hour on the HIGH setting.
Using a slotted spoon, transfer the chicken to a bowl. Pour the sauce into a large skillet and bring to a boil over medium-high heat. Cook until the sauce is reduced by half and thickens enough to coat the back of a spoon, about 7 minutes. Add the chicken to the sauce and toss to combine.
To serve, spoon over cooked rice, sprinkle with sesame seeds and scallions, and serve with orange slices, if desired.