Heat oven to 425° F. Whisk together the mayonnaise and miso in a small bowl; set aside.
Place the roast in the center of a large rimmed baking sheet and season with ¾ teaspoon each salt and pepper. Roast, fat-side up, until beginning to turn golden brown, 18 to 22 minutes. Toss the potatoes, bok choy, oil, and ¼ teaspoon each salt and pepper in a large bowl; add to the baking sheet.
Continue to roast until the potatoes are tender and an instant-read thermometer inserted into the thickest part of the roast registers 130° F (the roast will be medium-rare), 20 to 25 minutes more.
Transfer the roast to a cutting board and let rest for 15 minutes. Continue to roast the potatoes until crisp, 5 to 10 minutes more.
Slice the roast against the grain. Serve over the potatoes and bok choy with the scallions on top and the miso mayo on the side.
