Line a large rimmed baking sheet with foil and place on middle oven rack; preheat oven to 425°F.

Whisk 1 tablespoon oil, 2 tablespoons miso and 1 tablespoon soy sauce together in a small bowl. Arrange 4 salmon fillets, skin-side down, in the center of the preheated pan. Brush the salmon with half of the miso mixture, about 2 tablespoons. Set the remaining miso mixture aside.

Toss bok choy with the remaining 1 tablespoon oil and ¼ teaspoon salt in a large bowl and arrange, cut-side down, around the salmon. Roast until the bok choy stems are tender, about 8 to 10 minutes. Flip the bok choy; brush with the remaining miso mixture and return to the oven. Cook until the salmon flakes easily with a fork and the bok choy leaves are browned, about 5 minutes.

Divide 2 cups rice among 4 plates. Top with the salmon and bok choy.