In A Small Bowl, Dissolve Yeast In Water; Set Aside. In A Large Bowl, Combine The Flour, Sugar And Salt; Cut In Shortening Until Mixture Resembles Coarse Crumbs. Add Yeast Mixture. Combine Sour Cream And Eggs; Stir Into Flour Mixture Until A Smooth Dough Forms (Do Not Knead). Cover And Chill Overnight. Roll Dough To Fit A Greased 15-In. X 10-In. X 1-In Baking Pan. Form A Rim Around Edges; Set Aside. In A Skillet, Saute Onion In Butter Until Tender. Remove From The Heat; Stir In Remaining Topping Ingredients. Spread Over Dough. Bake At 350 Degrees F For 25 Minutes Or Until Golden Brown.