Rinse the rice and cook it in 480 ml (2 cups) of water: bring to a boil over high heat, cover, simmer on low for 15 minutes and rest 5 minutes.
Cut the ginger into thin matchsticks and slice the green onions into thin strips.
Set up a steamer: put a rack or steamer basket over 5 cm (2 inches) of water in a wide pot or wok and bring to a simmer over medium-high heat.
Lay the cod on a heatproof plate that fits inside, scatter with half the ginger and put the halved bok choy around it.
Set the plate on the rack, cover and steam for 8 to 10 minutes, until the cod is opaque and flakes easily, 63°C (145°F) in the thickest part.
Carefully lift out the plate. Scatter the rest of the ginger and the green onion over the fish.
Heat the canola oil in a small pan over high heat until it just smokes, about 1 minute, then pour it over the ginger and onion so they sizzle. Drizzle with the soy sauce and sesame oil and serve with the rice.