How should I prepare Bok Choy before I cook it? I just bought a pack of bok choy that I want to steam inside a bag along with some halibut.  How should I cut it up before putting it in the bag?  Would the preparation be different if I was cooking it for stir-fry (or something else)?

Should I separate the white stem from the leaf and eat both?  Won't the leaf cook much faster than the rib?  How much of the stem should be eaten?

Also, this isn't the "baby" bok choy, the shape is more like a head of romaine.
I've never tried steaming bok choy, but yes, the ribs and leafy bits cook at different speeds.

For stir frys I trim the bottom so the ribs come free, clean off any dirt that's gotten trapped in between, then stack up the leaves and cut the leaves from the ribs.  I slice the ribs into about 5mm / 3/16" slices, and add them towards the end; the leaves I cut into ribbons (perpendicular to the ribs), and stir them in at the last second, so they barely cook.  The center I just chop the top off the leafy bit, and slice the more rib-like bits a litte larger than the rest of the ribs (they're thinner, so cook faster).

As I've never tried steaming it, and I don't tend to cook in bags (except for silver turtles when camping), this is going to be a complete guess.  Now, it tastes fine al dente, so you could leave the ribs as large as I use for a stir fry, but en papillote tends to be fancier cooking, so I'd probably try for maybe 1/6" slices or a little thicker (maybe 2-3mm), and if you had a mandoline so they all came out even, that'd be great.  I'd still cut the leaves into ribbons, just because it's easier to eat than a huge intact leaf.

I'd assume that the leaves would cook at a similar rate as spinich, and the ribs closer to fennel, if that helps at all.