In a large skillet, cook onions in 1/2 cup butter over low heat for 10 minutes. Stir in the poppy seeds, salt, paprika and pepper; cook until onions are golden brown, stirring occasionally. Remove from the heat; set aside. , In a large bowl, combine the flour, cornstarch, baking powder and sugars. Cut in 1-1/4 cups butter until mixture resembles coarse crumbs. Melt the remaining butter. In a small bowl, whisk the eggs, milk, sour cream and melted butter. Make a well in dry ingredients; stir in egg mixture just until moistened. , Spread into a greased 10-in. cast-iron skillet or springform pan. Spoon onion mixture over the batter. Place pan on a baking sheet. Bake at 350° until a toothpick inserted in the center comes out clean, 35-40 minutes. Serve warm.