Mongoose gère les promesses :http://mongoosejs.com/docs/promises.html
Ainsi vous pouvez simplement :
app.post('/accounts', authenticate, (req, res) => {
var account = new Account({
name: req.body.name,
_owner: req.body._owner,
});
account.save()
.then((doc) => User.findOneAndUpdate(
{ uid: req.body._owner },
{ $set: { account: doc._id } },
{ new: true }
)
.then(() => doc);
}).then((account) => {
res.send(account);
}, (e) => {
res.status(400).send(e);
});
});