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 half c tepid water & 0.75 c rye flour and leave out overnight. Otherwise keep in fridge. (Good idea to feed the day before you're going to use it.)