Start by mixing together water and cornstarch in a small bowl until no clumps are left.
Add this slurry mixture alongside all other sauce ingredients into an 8 ounce mason jar or small freezer bag. Stir to combine.
Add chicken to a gallon-sized freezer bag and pour over half the sauce to coat chicken. Press out any air and freeze both chicken and sauce until ready to use.*
On cooking day, let chicken and sauce thaw for 8 hours or overnight.
Heat oil in a large pan or wok over medium-high heat. Use tongs to add marinated chicken to pan and discard any leftover marinade. Heat chicken on each side until just browned but not cooked all the way through, about 3-4 minutes.
Then, add vegetables to same pan and cook for 1-2 minutes until brightened in color.
Finally, pour over prepared sweet chili sauce. Turn the heat down to low and let simmer for 3 more minutes until slightly thickened.
Serve stir fry over rice and sprinkle with red pepper flakes and sesame seeds.
Video
Notes
*You can also skip the freezing and use the chicken later that day. Just allow the chicken to marinade for 6-8 hours.