Serve with white rice.

Adjust oven rack to lower-middle position and heat oven to 300 degrees. Process pear, soy sauce, sugar, sesame oil, and rice vinegar in food processor until smooth, about 30 seconds.

Pat beef dry with paper towels and season with salt and pepper. Heat vegetable oil in Dutch oven over medium-high heat until just smoking. Add half of beef and brown on all sides, 7 to 10 minutes; transfer to large plate. Repeat with remaining beef.

Add onion to fat left in pot and cook over medium heat, stirring occasionally, until softened, 5 to 7 minutes. Stir in garlic and ginger and cook until fragrant, about 30 seconds. Stir in sake and simmer, scraping up any browned bits, until reduced by half, about 2 minutes. Stir in pear mixture and browned beef and any accumulated juices and bring to simmer. Cover, transfer pot to oven, and cook until tender, 2 to 2½ hours.

Remove pot from oven, uncover, and place steamer basket on top of meat. Add bok choy to steamer, cover pot, and continue to cook in oven until bok choy is tender, 5 to 10 minutes.

Remove pot from oven. Remove basket of bok choy from pot and transfer bok choy to platter. Transfer beef to platter and tent with aluminum foil. Strain braising liquid through fine-mesh strainer into fat separator, discarding solids, then defat liquid.

Combine ¼ cup defatted liquid with cornstarch in bowl. Return remaining liquid to simmer in now-empty pot over medium heat. Whisk cornstarch mixture to combine, then whisk into simmering liquid in pot and cook until slightly thickened, about 2 minutes. Remove from heat and season with salt and pepper to taste. Serve short ribs and bok choy, passing sauce separately.