If your pan is big enough I would not even bother chopping it. Just layer the bok choy in the bottom of the pan as the recipe instructs. I would cook it for the original time of 30 minutes. If the stems aren't tender by then I would check every 10-15 minutes after your original 30 until the stems are tender. There's no way to tell an exact time because there are alot of variables to consider, but just keep checking every 10-15 minutes and you will be fine.

If you need to chop the choy I would chop it into 2 or 3 large pieces after trimming the stem- you want to keep the bok choy as large as you can. Again, you just need the pieces to fit in the bottom of the pan. Any cooking time you add will not hurt the meatballs as long as you aren't cooking the bok choy to mush.

When the stems are tender just double check that your meatballs are cooked through(open one up and see if there is any pink left).

Good luck!