In a medium bowl, combine cornstarch, brown sugar, 1/2 tsp salt, 1/2 tsp pepper and Chinese 5 Spice. Add chicken pieces and toss well.
In a large skillet or wok, heat oil over high heat. Heat for at least a minute or two to ensure the oil is very hot. You can test the oil by putting a wooden spoon, skewer or chop stick into the oil. If you see the oil form little bubbles around it, it's ready to go! Carefully drop in chicken pieces, spreading them in an even layer.
Fry chicken WITHOUT TOUCHING for 2-3 minutes on one side (check for a nice, golden brown, caramelized color). Then, carefully turn chicken (with tongs or a metal spatula) and cook about 2 minutes more. You want the chicken to have a nice, deep golden brown color. Just be aware the sugar will burn if you leave it too long :)
Remove chicken from pan and transfer to a paper towel-lined plate.
In the same pan over medium high heat, add onions and peppers. Season with remaining 1/2 tsp salt and 1/2 tsp pepper. Cook, stirring occasionally for 2-3 minutes. Add garlic and cook one minute more.
Add chicken back into pan. Cook for another minute, tossing until chicken, veggies and garlic are all well-mixed.
Top with green onions and a sprinkle more of salt and pepper, if you like. Serve over rice with a drizzle of sweet soy sauce, if desired.