2) Pour 2.5cm or more of water in a wok or skillet and bring to a boil over medium-high heat. Put the pak choy in a bamboo or collapsible steamer and cover. Set the steamer over the water, cover, and cook until just tender, about 3 minutes for baby pak choy, 4 minutes for medium pak choy.
3) Meanwhile, heat a large nonstick skillet over high heat. Season the salmon with the salt and pepper, lay the fish rounded side down in the pan and cook until golden and crisp on one side, about 2 minutes. Reduce the heat to medium, turn and cook the fish on the remaining side to the desired degree of doneness, about 6 to 7 minutes for medium-rare, 8 to 9 minutes for medium, and 11 to 12 minutes for fish cooked through.
4) Divide the salmon and pak choy among 4 plates and serve each drizzled with about 50ml of the ponzu. Pass extra sauce at the table. Store leftover ponzu in airtight container in refrigerator for 1 week.