In a large bowl, combine the minced Ground Beef and Ground Lamb. Add finely chopped or grated Garlic, a generous pinch of Sea Salt and freshly ground Black Pepper.
Dissolve the Baking Soda in the Water and pour it into the meat mixture; this helps keep the cevapcici tender. Mix thoroughly with your hands until the mixture becomes sticky and well combined. Cover and refrigerate for at least 1–2 hours or preferably overnight to develop flavor.
When ready to cook, wet your hands slightly and shape the meat into small finger-sized sausages (about 8–12 pieces, depending on size). Place them on a tray and chill for 15–30 minutes so they hold their shape.
Preheat a grill or heavy skillet over medium-high heat and brush with Neutral Oil. Cook the cevapcici for about 8–12 minutes, turning occasionally, until evenly browned and cooked through with a nice char on the outside.
While the meat cooks, thinly slice the Yellow Onion and sauté in a little more Neutral Oil over medium heat until soft and golden, seasoning with a pinch of salt and pepper to taste.
Warm the Lepinja briefly on the grill or in the oven until pliable and lightly toasted.
To serve, open the warm lepinja, spread a generous spoonful of Ajvar inside, add several cevapcici and top with the caramelized onions. Serve immediately with extra ajvar on the side.
