In a bowl, sift together flour, baking powder & salt; rub in butter to form fine breadcrumbs. Stir in yogurt/sage mixture as well as sauteed onion. Turn out onto a floured surface & knead very lightly. Divide dough into 4 or 6 equal pieces. Form into balls & lay on prepared baking sheet. Flatten to 3/4″ thickness; brush tops with beaten egg. Bake about 10 minutes or until risen & golden.