Bok choy
Chinese leaf cabbage with fleshy pale stalks and dark green leaves, mild with a faint mustardy note.
Bok choy is sold in larger supermarkets and at markets, and reliably in Asian grocers, often also as small baby bok choy. Stalks and leaves cook at different speeds, so the stalks go into the pan first.
Substitute:
Swiss chard comes closest in bite and structure but tastes earthier; napa cabbage is softer and more watery, and spinach replaces only the leaves, not the crisp stalks.