Place the onion, garlic and chili in a food processor and process briefly.
Add the ground beef, parsley, oregano, paprika, cumin, salt and pepper and blend to a paste.
Transfer the mixture to a bowl, cover and refrigerate 30 minutes. With damp hands, take an egg-sized piece of the mixture and press it into a long sausage shape on a skewer. (Skewers made from rosemary twigs may be used to add extra fragrance.)
Broil the brochettes under the broiler or on a grill, turning frequently, 6 to 7 minutes, until well browned on the outside but still moist inside. To serve, spoon yogurt over the brochettes and sprinkle with chopped scallions.