In a bowl, whisk soy sauce, sesame oil, rice vinegar, garlic, ginger, sweetener, and red pepper flakes.
Add sliced beef and marinate at least 30 minutes (overnight for best flavor).
Heat a skillet over high heat, add marinated beef, and cook 2–3 minutes until browned and caramelized.
In another pan, sauté cauliflower rice in avocado oil until tender, about 5 minutes. Season lightly with salt.
Assemble bowls: cauliflower rice on the bottom, beef on top, then garnish with avocado, green onions, sesame seeds, and optional egg or kimchi.