In a large mixing bowl, combine the ground beef, ground lamb, finely chopped onion, minced garlic, salt, ground black pepper, paprika, and baking soda.
Gradually add the sparkling water while mixing the ingredients thoroughly with your hands. The mixture should be well combined and slightly sticky.
Cover the bowl with plastic wrap and refrigerate for at least 1 hour, preferably overnight, to allow the flavors to meld.
Divide the meat mixture into small portions and shape each portion into a sausage-like shape, about 3-4 inches long and 1 inch thick.
Place the shaped ćevapi on a tray lined with parchment paper.
Preheat your grill to medium-high heat.
Brush the grill grates with vegetable oil to prevent sticking.
Place the ćevapi on the preheated grill.
Grill the ćevapi for about 4-5 minutes on each side, or until they are cooked through and have a nice char on the outside.
Use tongs to turn the ćevapi and ensure they cook evenly on all sides.
Warm the pita breads or flatbreads on the grill for a minute or two.
Place a few ćevapi in each pita or flatbread.
Top with finely chopped onion and a dollop of ajvar or sour cream.
Garnish with fresh parsley if desired.
Enjoy your meal!