Prep the chicken: Pat the chicken dry and cut into bite-size pieces. Season lightly with salt and pepper.
Set aside while you make the sauce.
Make the keto teriyaki sauce: In a bowl, whisk 1/2 cup tamari (or 3/4 cup coconut aminos), 2–3 tablespoons keto sweetener, 1 tablespoon rice vinegar, 1 teaspoon sesame oil, 2–3 cloves minced garlic, and 1 tablespoon grated ginger. Taste and adjust sweetness or saltiness.
Heat the pan: Add 1 tablespoon avocado oil to a large skillet over medium-high heat. When hot, add the chicken in a single layer.
Don’t overcrowd the pan.
Sear the chicken: Cook for 3–4 minutes without moving, then flip and cook another 3–4 minutes until browned and cooked through. Work in two batches if needed. Transfer cooked chicken to a plate.
Reduce the heat and deglaze: Lower the heat to medium.
If the pan is dry, add a splash of water. Pour in the teriyaki sauce and stir, scraping up brown bits for extra flavor.
Thicken the sauce: Sprinkle a tiny pinch (about 1/8 teaspoon) of xanthan gum over the sauce while whisking constantly to avoid clumps. Let it simmer 1–2 minutes.
Add another small pinch only if needed. The sauce should coat the back of a spoon.
Combine: Return the chicken and any juices to the pan. Toss to coat and simmer 1–2 minutes so the chicken absorbs the sauce.
Finish: Stir in a pinch of red pepper flakes if you like heat.
Taste and adjust seasoning—more sweetener for sweeter, more tamari for saltier, or a splash more vinegar for brightness.
Garnish and serve: Top with sesame seeds and sliced green onions. Serve over cauliflower rice or with steamed or sautéed low-carb vegetables.