In a large measuring cup, whisk together the coconut aminos (or soy sauce), mustard, brown sugar substitute, minced ginger, and orange extract.
Place the beef in a shallow bowl or resealable bag. Pour in half the marinade, turning the beef pieces to coat evenly. Cover or seal and refrigerate for at least 3 hours. Reserve the remaining marinade for the sauce.
Remove beef from the marinade and pat dry with paper towels to remove excess liquid. Discard used marinade.
Heat oil in a large skillet over medium-high. Add beef in a single layer, working in batches if necessary. Stir-fry 3-4 minutes per batch, or until browned and cooked through. Set cooked beef aside.
Add the bell pepper and green onion to the hot skillet, reserving some green onions for garnish. Stir-fry 2-3 minutes, until peppers are tender-crisp. Remove to the plate with the beef.
In a small saucepan, combine the reserved marinade and water. Bring to a low boil. Sprinkle in the xanthan gum. Cook while whisking continuously for about 1 minute until the sauce thickens.
Return beef and vegetables to the skillet. Pour sauce over and toss to coat evenly.
Serve hot, garnished with sliced green onions if desired.