Mix all sauce ingredients together in a small bowl with the cornstarch and 2 green onions. Set aside.
Pat the beef dry with clean paper towel.
Heat a large skillet or wok over medium high heat, add a drizzle of oil, about 1 tablespoon. Once shimmering, add the beef, working in batches so you don’t crowd the pan. Cook until lightly seared, about 3 minutes, it’s ok if it’s not cooked through at this point. Remove to a plate.
Add the broccoli and sliced onion to the pan and saute until brown and crisp tender, about 3 minutes, adding another drizzle of oil as needed.
Add the sauce ingredients to the pan, simmer until the broccoli is cooked to your liking. Add beef and any accumulated juices on the plate, along with the green onion, cook an additional 1 minute until the beef is warmed and just cooked through. Taste and adjust seasoning with salt and pepper as desired. Serve immediately along with rice, cauliflower rice or noodles.