Mix yeast, sugar, 1 t salt, and 1/2 cup flour. Blend in shortening and warm water. Beat for 2 minutes. Add enough flour to make a soft dough. Knead dough until smooth and elastic, about 5 minutes. Place dough in a lightly greased bowl. Cover and let dough rise in a warm place 1/2 hour. Pat dough into a lightly greased 12-inch pizza pan or onto a lightly greased baking sheet. Press up edges to make a slight rim. Fry bacon until crisp. Remove from grease and drain on absorbent paper. Add onions to bacon grease; cook slowly until tender but not brown. Sprinkle onion, bacon, cumin, 1/2 t salt and pepper over dough. Bake at 400 Degrees F. for 20 minutes. Blend egg yolk and sour cream. Pour over onions. Bake for 10 to 15 minutes longer or until golden brown and sour cream is set. Serve warm or at room temperature.