Preheat oven to 350˚F. Over low heat sauté onions in 1/2 cup butter for 10 minutes. Add in poppy seeds, salt, paprika, and pepper and stir to combine. Remove pan from heat and set aside.
In a large bowl mix together flour, cornstarch, baking powder, and sugar. Cut in 1 1/4 cups butter to form a loose and crumbly dough.
Melt the rest of the butter. In a separate bowl whisk together eggs, milk, sour cream, and melted butter. Make a well in the dry ingredients and pour the wet mixture into the dry. Stir only until just combined.
Pour batter into a greased 10” springform pan. Top with caramelized onions. Bake for 35-45 minutes or until knife inserted in center comes out clean. Allow to cool for at least 5 minutes before releasing from the pan.
