Place beef in freezer for 15 minutes to firm up for easier slicing.
Slice beef thinly against the grain.
Toss beef with 2 tbsp cornstarch until coated.
Whisk soy sauce, coconut aminos, honey, rice vinegar, sesame oil, red pepper flakes, water, and cornstarch in a bowl.
Heat oil in a large skillet or wok over high heat.
Add beef in a single layer and cook 1–2 minutes until browned.
Flip beef and cook another 1–2 minutes until lightly browned.
Remove beef from pan and set aside.
Reduce heat to medium and cook garlic and ginger for 30 seconds.
Add bell pepper and broccoli if using and stir-fry 2–3 minutes.
Stir sauce and pour into pan.
Simmer until sauce thickens, about 1–2 minutes.
Return beef and green onions to pan and toss until coated and heated through.
Serve immediately over cauliflower rice or brown rice.
Garnish with sesame seeds and extra green onions.