12ounces baby bok choy or other Asian greens (like tatsoi, gai lan, pea shoots, or water spinach)
8 to 12ounces fish fillets (like salmon, cod, bass, or rockfish; or use chicken breasts or firm tofu steaks)
Soy sauce and lemon wedges for serving
Instructions
Put the butter, zest, and sesame oil in a small bowl; sprinkle with a little salt and lots of black pepper and stir with a fork to combine. Let sit.
Peel the garlic and ginger and slice thinly however you’d like. Put in a 10-inch skillet that has a tight fitting lid. Pour in the sake (or water) and 1 cup water and sprinkle with salt and pepper. Bring the liquid to a boil, adjust the heat so it bubbles gently, and cover. Simmer until garlic and ginger are tender, 5 to 10 minutes.
Trim the greens and slice the stalks and leaves; rinse them well in a colander, tossing to remove any grit. When the aromatics are ready, add the greens and stir. Put the fish (or whatever) on top skin side down, sprinkle lightly with salt and adjust the heat so the liquid bubbles steadily but gently.
Cover and cook, undisturbed, for 5 minutes before nicking the fish with a small knife to peek inside for doneness. Keep checking every couple minutes; you want the thickest part to be slightly underdone. When it’s ready, put the butter on top of the fish, cover, and remove from the heat to rest for about 3 minutes. Taste and adjust the seasoning. You can serve the fillets whole or portioned, or if you want to remove the skin (which will be sort of rubbery), do that while breaking the fish into big chunks. Pass soy sauce and lemon wedges at the table.