Easy refrigerator pickled beets

From my mom, who can’t remember where she got it

2 lbs beets
1.5 c cider vinegar
1.5 T dry mustard
1/2 t salt
1.25 c sugar
2 medium onions, sliced thinly
2 t celery seeds

cook beets until tender, & drain, reserving 1 c cooking liquid; slip off skins & slice
heat vinegar & reserved water to a boil
mix mustard, salt, sugar; add to vinegar & let boil again
arrange beets & onions in layers in canning jar; add celery seeds & cover w/vinegar mixture
cool, refrigerate, use w/in 2 days