The best meatballs, contrary to what many think, are not purely meat. A meatball with no filler tends to be tough and dense as meat proteins crosslink and tighten up during cooking. It's by adding something—say, a mixture of bread and milk called a panade, or other cooked grains or legumes—that you get the tender results of the ideal meatball.
In this recipe from the superb book Tender, Nigel Slater goes running with this idea, using not only cracked wheat to bulk out the meat, but also grated raw beets. I was skeptical but drawn to their incredible rich, red color. The result was absolutely wonderful, with the earthy beets marrying perfectly to the flavor of lamb, with plenty of freshness from the herbs. It made a small amount of ground meat go much, much further.