Include lots of onion and some minced bell pepper. Carrot, too, will add flavor. Saute the vegetables well, because browning adds flavor, before adding them to the mix. A generous amount of black pepper and allspice will help. Balsamic vinegar will add deeper flavor than lemon zest, though anything acidic is good when no salt is used. Rather than meatballs, form into 3/4" thick patties, saute over medium-low heat, 10 mi9 per side. Again, more browning that way.
I haven’t added salt to my food in decades, though of course some ingredients, like cheese, are salty. I adore Trader Joe’s 21-Seasoning Salute, which is salt-free but does wonders for seasoning just about everything I cook.
