Dissolve yeast and sugar in water. Combine milk, shortening and salt; stir into yeast mixture. Add enough flour to form a soft dough. Turn onto a floured surface; knead until smooth and elastic, about 3 minutes. Place in a greased bowl, turning once to grease top. Cover and let rise in a warm place until doubled, about 1-1/2 hours. , Punch down dough; press into a greased 13x9-in. baking pan. Cover and let rise in a warm place until doubled, about 45 minutes., Preheat oven to 425°. Combine cheese, egg, milk and onion; spread over dough. Sprinkle with poppy seeds. Bake 15-20 minutes. Cut into squares; serve warm.