Prepare the slow cooker liner by spritzing with nonstick cooking spray.
Season both sides of the steak with salt and black pepper as desired.
Heat a heavy skillet over medium-high heat until the skillet is hot. Add the oil and then the steak. Brown the steak on both sides but do not cook through. Transfer the steak to the slow cooker liner.
Deglaze the bottom of the skillet with water, scraping to remove any cooked on pieces. Pour the liquid over the steak in the slow cooker.
Mix together the garlic powder, tomatoes, liquid aminos, and vinegar. Pour over the steak.
Arrange vegetables on top of the steak, seasoning with salt and black pepper if desired.
Cover the slow cooker and cook on high for 3 to 4 hours or low for 6 to 8 hours.
If a thicker sauce is desired, remove the steak and vegetables from the slow cooker. Stir in the xanthan gum and re-cover, allowing the sauce to thicken slightly. Add the meat and vegetables back to the slow cooker prior to serving.