Put water in a pot; add long beans, chayote, ginger, and green onion. Bring the water to a boil, place a lid on the pot, and reduce heat to medium. Continue cooking until the vegetables are tender, about 8 minutes.
Gently drop tuna steaks into the water; add bok choy and tomato. Return cover to pot, reduce heat to low, and cook until the tuna is cooked through, about 20 minutes.
Remove pot from heat; cool 5 minutes before serving.