In a large bowl, place the ground beef, ground lamb, salt, and baking soda. Combine all very well.
Cover the bowl and put it in the fridge and let rest for one full day.
The next day, 2-3 hours before cooking; add the grated onion and mix all well. Put back in the fridge and let rest until it's time to cook.
Take walnut-size pieces of ground meat and roll into small balls, and press slightly in between your palms, or give it a finger shape.
Place the meatballs on the grill or on a non-stick pan. And then cook the one side first and then flip them over and don't use any vegetable or olive oil.