Chevap chi chi (Serbian sausage)
Yes, raw onion, bread, and beer are definitely traditional accompaniments. I prefer the onions diced.
Other things that go well:
--Cabbage salad (green cabbage, vinegar, oil, salt and pepper)
--Roasted sweet red peppers--try roasting/charring a hot pepper, like a serrano, too. :)
--Shopska salad: onion, tomato, cukes, banana pepper, feta
Also, some parts of the Balkan serve cevapcici with "bukovska piper." It's dried, roasted, crushed red pepper. You can have this hot or mild. You literally take a cooked cevap and dunk it, coating it in the pepper. It offsets the meat perfectly.
Technically a cevap isn't a sausage. It's more like a lula kabob---minced/ground meat that's shaped like a log and then grilled (sans skewer).
Raise a glass of beer and say "Ziveli!" :) Enjoy 'em. They're great.