In a bowl, whisk together 1 tbsp olive oil, balsamic vinegar, honey, Dijon mustard, garlic, salt, and pepper.
Add sliced chicken, toss to coat, and let marinate for 10 minutes (or up to 30 minutes for deeper flavor).
Heat 1 tbsp olive oil in a large skillet over medium-high heat.
Add chicken (reserving marinade) and cook for 4–5 minutes until browned. Remove and set aside.
In the same skillet, add peppers, broccoli, and onion. Stir-fry for 3–4 minutes until crisp-tender.
Sprinkle with Italian herbs and red pepper flakes.
Return chicken to the skillet. Pour in reserved marinade and cook for 2–3 minutes until sauce thickens slightly.
Top with fresh basil or parsley. Serve over rice, quinoa, or noodles.