Perpetual Spinach (Leafbeet) Chard
Not all chards are the familiar "Swiss" type with broad ribs and glossy leaves. Perpetual spinach has small stems, and softer, more tender leaves. They look and taste much like spinach, with tender spinach "mouthfeel" when cooked. They are more heat-resistant than spinach, and make a deep taproot that gives them tremendous vitality and drought-resistance. I have seen these live for several years in the garden, pumping out tender green leaves through 100-degree heat, with very minimal care. Not as cold-hardy as kale, but faster-growing and much more heat tolerant.
Perpetual Spinach produces more food in a small space than anything else I know. Overwinters with no problem for me in zone 7, and have heard of them surviving down to zone 5. For longest life, give them good drainage, lots of compost, mulch them (but not touching the crowns) and remove the flowering stalks before they stretch up and mature seed.