Replacement for mustard seeds?

Oh, you're wonderful, Panfusine! Thank you.

Thanks for the compliments boulangere... Totally agree with her... Leave the mustard out.. the way the spices work is that they split in the hot oil releasing their own characteristic aroma.. powdered mustard would quickly burn up in this level of heat giving the dish a burnt bitter note..if you don't have whole mustard seeds, dont fret, there is enough flavor to be had from the ground spices..

I wish the recipe instructions were a bit clear though, the rest of the spices are powders and there is no clear instruction on how to treat the mustard seeds, I'd say, in step 3, once the remaining oil is hot, first add the mustard, wait for it to sputter & then lower the heat & add the remaining spice mix, before adding the greens to cook

You'll get much better advice from Panfusine or Pauljoseph, but in the interim, I would just say leave them out. Mustard powder is going to give you a very different texture and flavor, so I wouldn't suggest that route.