Heat sunflower oil in a large pan over medium heat. Add chicken breast pieces and cook until browned and cooked through, about 5-7 minutes. Add chopped onion, broccoli, and minced garlic, stir-frying for another 3-4 minutes until the vegetables are tender.

Pour in the soy sauce and toss everything together. Cook for an additional minute to combine flavors and serve immediately.

Serve the stir-fry hot over rice or noodles. Garnish with sesame seeds if desired. Leftovers can be stored in an airtight container in the fridge for up to 2 days.