Whisk the sauce ingredients.
Whisk the vegetable oil, sesame oil, ginger, and garlic. Add the strips of top sirloin and turn to coat.
Prepare the grill for direct cooking over high heat (450° to 550°F).
Place a grill-proof wok on the cooking grate, close the lid, and preheat it for about 10 minutes. Bring the sauce, top sirloin mixture, bell peppers, onion, and cilantro to the grill and keep close by.
When the wok is smoking hot, add the top sirloin mixture, separating the meat as you add it to the wok. Grill over direct high heat, with the lid open, until the meat starts to brown and releases easily from the wok, about 1 minute, stirring once after 30 seconds. Add the bell peppers and the onion all at once. Stir to combine and cook until the vegetables turn a shade brighter, about 2 minutes, stirring frequently. Add the sauce, stir for 10 seconds, close the lid, and cook until the sauce comes to a boil, about 1 minute. Simmer until the sauce is thick enough to coat the vegetables, about 1 minute more. Stir in the cilantro. Remove the wok from the grill and serve the stir-fry immediately over warm rice.