1. Preheat oven to 375 degrees Fahrenheit (190 degrees Celsius).
2. Sauté onions in butter and oil for about five minutes over moderate heat.
3. Add mushrooms, if desired, salt, and black pepper and sauté another five to eight minutes.
4. Meanwhile, chop potatoes finely in the food processor, using the knife blade. Drain and set aside.
5. Combine onion mixture with potatoes in a large bowl. Mix in eggs and sour cream.
6. Lightly oil the bottom of a 9×13-inch (23×33-centimeter) pan (glass recommended).
7. Sprinkle bottom with cornflake crumbs.
8. Pour kugel mixture into pan and spread evenly. Dust top with paprika and dill.
9. Bake one to one and a half hours, uncovered, until crisp on top and potatoes are soft.
Every oven differs, but mine needed almost one and a half hours to cook this casserole. Serve with applesauce and sour cream, sprinkled with cinnamon, if desired.
This can be made for Passover using any crumb topping. Try Heaven and Earth Veggie Coating crumbs, Haddar Passover Pretzel Crumbs, or Gefen Nut Crumbs.