Thinly slice the sirloin steak against the grain. In a bowl, combine steak with soy sauce, oyster sauce, cornstarch, sesame oil, ground ginger, garlic powder, and black pepper. Marinate for at least 30 minutes.

Chop the onion, bell peppers, broccoli, carrots, snow peas, and mushrooms into bite-sized pieces.

In a separate bowl, whisk together the chicken broth, soy sauce, oyster sauce, brown sugar, cornstarch, sesame oil, and red pepper flakes.

Preheat the Blackstone griddle to medium-high heat, then add vegetable oil.

Sear the marinated steak on the griddle for 2-3 minutes on each side until browned.

Remove the steak and set aside. Stir-fry onions, bell peppers, carrots, and broccoli for 5-7 minutes until tender-crisp.

Add minced garlic and ginger, cook for 1 minute.

Stir in snow peas, mushrooms, and water chestnuts, stir-fry for 2-3 minutes.

Pour the prepared sauce over the vegetables and simmer for 1-2 minutes.

Return the steak to the griddle, mix with vegetables and sauce, and serve over cooked rice.

Garnish with sesame seeds and green onions before serving.