Heat olive oil in a large skillet or wok over medium-high heat.
Add sliced chicken sausage and cook for 3-4 minutes until lightly browned.
Add onion, bell peppers, and broccoli. Stir-fry for 4-5 minutes until vegetables are tender-crisp.
Add minced garlic and cook for another 30 seconds until fragrant.
Stir in soy sauce, oyster sauce, and chili flakes if using. Mix well.
Season with salt and pepper to taste and serve hot.