In a small bowl, whisk together the soy sauce, water, sweetener, minced garlic, and grated ginger to create the marinade sauce.
Heat 1 tablespoon of avocado oil in a large wok or heavy skillet over high heat.
Add the beef slices in a single layer and sear until browned and crisp at the edges, approximately 2 minutes per side, then remove beef from the pan and set aside.
Add the remaining tablespoon of oil to the same pan and add the shredded cabbage. Stir-fry for 3 to 5 minutes until the cabbage is tender-crisp.
Return the cooked beef to the pan with the cabbage and pour the sauce mixture over the ingredients.
Slowly sprinkle the xanthan gum over the liquid while stirring constantly to prevent clumping until the sauce thickens and coats the beef.
Drizzle with toasted sesame oil and toss in the sliced green onions and red pepper flakes.
Serve immediately as a standalone dish or over cauliflower rice.