Cut the chicken breast into bite-sized pieces and toss in a bowl with the cornstarch until lightly and evenly coated.
In a small mixing bowl, whisk together the tamari, rice vinegar, honey, minced fresh ginger, minced garlic, and red pepper flakes to create the sauce.
Heat the avocado oil in a large non-stick skillet or wok over medium-high heat.
Add the chicken to the skillet and cook, stirring occasionally, until the pieces are golden brown and cooked through, about 6 to 8 minutes.
Remove the chicken from the skillet and set aside; add the broccoli florets and sliced red bell pepper to the same pan with a splash of water.
Cover and steam-fry the vegetables for 2 to 3 minutes until they are crisp-tender and bright in color.
Return the chicken to the skillet and pour the prepared sauce over the mixture, tossing constantly for 1 minute until the sauce thickens into a glossy glaze.
Serve the stir-fry immediately over a bed of warm brown rice and garnish with sesame seeds.