Nothing says Passover like matzo ball soup. The key to these tender, flavorful matzo balls is a hint of schmaltz (chicken fat), a little seltzer and an extra-long simmer. If schmaltz is not readily available at your market, ask the butcher or seek out a local meat market. Or, if you have chicken on hand, remove the skin and cook it over low heat to render the fat. If matzo meal is unavailable, look for whole matzo crackers and grind them at home in your food processor.