Preheat oven to 425. Grease 10-inch springform pan. Seasoning Mix: In small bowl combine poppy seeds, paprika, salt, onion powder, marjoram, peppers and dillweed. Set aside. Melt 4 tablespoons butter in large skillet over medium high heat. Add chopped onions and cook until golden (10-15 minutes). Add 4 teaspoons of the Seasoning Mix and cook until fragrant, 30 seconds. Remove from heat and set aside. In medium bowl whisk eggs, sour cream, 2 tablespoons melted butter and dark brown sugar. Set aside. Combine flour, cornstarch, sugar, baking powder and remaining Seasoning Mix in food processor. With machine on add 1/2 cup cold butter, cut up; process until mixture resembles fine crumbs. In large bowl, whisk 3 large eggs and 1/2 cup milk. Stir in flour mixture just until moistened. Spread batter evenly into prepared pan. Spread onion mixture evenly over batter. Top with a layer of sour-cream mixture. Bake 25 to 30 minutes until top is golden and bubbly. Cool 10 minutes, then remove sides of pan. Cut cake into wedges and serve warm or at room temperature.