Sprinkle yeast over tepid water. Beat in flour till no lumps remain. Peel and halve onion; add to starter. Cover loosely with cloth & let stand at room temp for 24 hrs.
Remove onion. Beat in tepid water & flour. Cover with cloth and let stand for 24 hrs more. The starter should now be pleasingly sour-smelling, almost beery and bubbly.
Feed once a week with 1/2 c tepid water & 3/4 c rye flour and leave out overnight. Otherwise keep in fridge. (Good idea to feed the day before you're going to use it.)
Source: Robert S. Benjamin MD. From the recipe files of Sylvia Steiger, GEnie THE.STEIGERS, CI$ 71511,2253, GT Cookbook echo moderator at net/node 004/005