Heat the avocado oil in a large skillet over medium-high heat. Add the onion and saute, stirring occasionally, until it turns translucent, about 5 minutes. Stir in the garlic and continue cooking for another 1 to 2 minutes.
Scoot the onion and garlic off to one side of the skillet. Add the ground beef to the skillet and allow it to brown for 2 minutes before flipping it and allowing the other side to brown another 2 minutes. Use a spatula to chop up the beef and stir it into the onion.
Combine the coconut aminos, water, sesame oil, ginger, cider vinegar, tapioca flour, and black pepper in a small blender and blend until smooth (Note: if you don’t have a small blender, you can simply whisk everything up in a bowl - just be sure to grate or finely chop the ginger first).
Pour the sauce into the skillet with the beef and bring to a full boil. Reduce the heat to a gentle boil and continue cooking until the sauce has thickened, about 8 to 10 minutes.
Serve Mongolian Ground Beef with your choice of sides - I always go with steamed brown rice and veggies. To keep it grain-free, serve with cauliflower rice or zucchini noodles.