In a glass or ceramic bowl, or jar that has been scalded, combine flour and yeast, add water and blend well. Add onion slice, cover with plastic wrap and pierce plastic with fork to release gases. Place in a warm, draft-free place, at an even 85F for 3 days. ; stir several times daily. Remove onion slice and refrigerate until ready to use. Onion imparts a strong flavor and this starter can only be used with compatible rye recipes; rye starter without onion can be used in any rye sourdough recipe. To replenish, always use rye flour. If starter does not seem bubbly after replenishing and standing 12 hrs, sprinkle 1 t yeast over and stir in well. Like whole wheat starter, you may have to plan longer rising times for dough made with this one.