In a mixing bowl, toss chicken pieces with 2 tablespoons cornstarch, salt, and pepper until evenly coated.
In another mixing bowl, whisk together honey, soy sauce, minced garlic, rice vinegar, sesame oil, 1 tablespoon cornstarch, water, ginger, and red pepper flakes. Set aside.
Heat vegetable oil in grill or cast iron skillet over high heat until smoking. Add chicken in a single layer and cook for 3-4 minutes per side using tongs until golden brown and cooked through. Transfer to a plate.
In the same skillet, add broccoli, bell pepper, snap peas, and carrots with 2 tablespoons water. Stir-fry for 3-4 minutes on cutting board until vegetables are crisp-tender.
Return chicken to skillet. Pour honey garlic sauce over everything and toss to coat.
Cook for 2-3 minutes, stirring constantly with tongs, until sauce thickens and becomes glossy, coating chicken and vegetables.
Remove from heat. Taste and adjust seasoning with more soy sauce or honey if needed.
Serve immediately over hot rice. Garnish with sliced green onions and toasted sesame seeds.