Season chicken with salt and pepper.
Place the cornstarch in a shallow bowl or pie plate. Place the eggs in a separate shallow bowl. Working in two batches, toss the chicken pieces in cornstarch and then coat with the egg.
In a large skillet, heat the oil over medium-high heat. Again working in two batches, brown the chicken, turning it so that all sides are browned. Then brown up the sliced onion.
Place the chicken & onion & other vegetables in the slow cooker.
In a small bowl, whisk together the sauce ingredients and pour evenly over the chicken. Turn the chicken to ensure each piece is coated.
Place chicken in. Pour the sauce over the chicken and stir to coat.
Cook on low for 6 hours or high for 3 hours