Process the onion in a food processor for 10 to 15 seconds and turn out into the center of a tea towel. Gather up the ends of the towel and squeeze until almost all of the juice is removed. Discard juice.
Mix onion, lamb, beef, garlic, marjoram, rosemary, salt, and pepper in a food processer or by hand. Form into meatballs and grill to an internal temp of 175
Serve with pita bread and tzatziki sauce
Adapted from the Food Network