Put oil into steamer bag. Add salt, pepper, parsley and seasonings. Close bag and mash bottom of bag to dissolve and distribute seasonings into oil, add a bit more oil if you need to.
Add potatoes, onion and about a tablespoon of butter. Seal bag and cook on high in microwave for 7 minutes. If you are using a regular ziploc bag, just close the bag half-way to allow for steam to release.
Carefully shake bag, cook 7 more minutes. Shake again and continue to cook for 5 more minutes.
Open bag (careful, it's steamy) and pour potatoes into serving bowl, add cheeses and butter, carefully - so as not to mash potatoes - mix to melt cheeses and butter into potatoes.
Top with bacon pieces, sour cream or your choice of toppings.