Whisk all of the ponzu sauce ingredients in a small bowl and set aside.
Pour 1-inch or more of water in a wok or skillet and bring to a boil over medium-high heat. Put the bok 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 bok choy, 4 minutes for medium bok choy.
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 1 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.
Divide the salmon and bok choy among 4 plates and serve each drizzled with 2 to 3 tablespoons of the ponzu. Pass extra sauce at the table. Store leftover ponzu in airtight container in refrigerator for 1 week.
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