In a large bowl, combine the ground beef, ground lamb, garlic, onion, salt, and water.
Mix thoroughly by hand until the ingredients are evenly incorporated.
Cover and refrigerate overnight.
The next day, pass the mixture through a meat grinder twice for a smoother texture.
Leave the mixture at room temperature for 1–2 hours, then return it to the refrigerator.
Add the baking soda and mix thoroughly.
Shape the mixture into small sausage-shaped Ćevapi, about 3–4 inches long.
Preheat a grill to medium-high heat.
Grill the Ćevapi for 8–10 minutes, turning occasionally, until browned and cooked through.
Serve immediately with chopped onions, ajvar, kajmak, lepinja bread, and feta cheese if desired.