What's the best way to seal the edges? I am constantly beset by gaping halves no matter how much I mash them together

Word to the wise: resist he urge to overfill. Here as with ravioli. Less is more.

Btw.... "constantly beset by gaping halves" is going to find its way into my daily conversation from now on. Thank you.

When I egg wash, I only brush one side since egg wash doesn't stick as well to more egg wash. Then I make sure I don't stretch the dough, if you stretch it, no amount of crimping or egg wash will prevent it from shrinking back, especially a relatively lean dough like this.

I would say "don't overfill with cheese" as tempting as that can be. To keep the seal intact, maybe brush a little beaten egg along the edge and then crimp with a fork. Also important, make small slashes or holes on the top of the pocket to let the steam vent out. This would help you avoid the 'blowouts' that leave gaping halves.