You can totally cook lettuce and eat it. It's healthy and tastes just fine. A quick blanch in boiling water is all it takes.
"Horta" is any green leaves you like, blanched and served with oil and lemon. Sure, it tends to be Dandelion or Rocket or whatever, but I don't think it's specified by the greek ya-yas, its whatever is to hand.
I think cabbage is a universal in climates with a real winter: its hardy, it grows into the colder months if you let it, its viable for pickling and salting, and flatulance aside is great nourishment. Shame that kids hate it, but eh, the ones which survive fatten up fast on it, so there's protein latent in cabbage if you need it.
Cabbage in school meals in the 60s was indeed served in an ice-cream scoop puddle next to the mince and tatties (or haggis, being scotland)