In a large bowl or zip-top bag, combine chicken cubes with cornstarch, salt, and pepper. Toss until chicken is thoroughly and evenly coated.
Heat vegetable oil in a large skillet over medium-high heat. Sear chicken in batches for 2 minutes per side until a light golden crust forms. Note: The chicken does not need to be cooked through at this stage.
Transfer the seared chicken pieces into the slow cooker insert.
In a separate mixing bowl, whisk together orange marmalade, soy sauce, rice vinegar, sesame oil, ginger, garlic, and red pepper flakes until well combined.
Pour the sauce mixture over the chicken in the slow cooker and stir gently to ensure every piece is coated.
Cover and cook on Low for 3 to 4 hours, or on High for 1.5 to 2 hours, until the internal temperature of the chicken reaches 165°F (74°C).
Stir the chicken once more to incorporate the thickened sauce. Garnish with sliced green onions and sesame seeds before serving.