Preheat an indoor grill, and coat with cooking spray. Place the chicken strips on the grill, and cook for about 7 minutes. Allow them to cool slightly, then cut into cubes.
While the chicken is cooking, heat the oil in a large skillet over medium heat. Place the onion rings in the skillet, and cook for a few minutes. Add the red and yellow pepper, baby corn, and the stir-fry vegetables. Increase the heat to medium-high; cook and stir for about 15 minutes.
Sprinkle in the sugar and salt. Stir in the chicken. Dissolve the cornstarch in water, and pour into the skillet along with the soy sauce. Cook and stir until the sauce thickens.