"Substitute Grana Padano cheese for Parmesan if desired. You can cube butternut squash a food processor with the pommes frites (french fry) cutter and then just pulse it one or two times with a regular blade."
1Preheat oven to 425 degrees F (220 degrees C). Grease a 9x13-inch casserole dish.
2Heat olive oil in a skillet over medium heat; cook and stir onions in the hot oil until softened, about 10 minutes. Stir in squash and garlic; season with salt. Cook squash mixture, stirring occasionally, until squash is slightly tender, about 10 minutes. Add water if mixture gets too dry. Transfer squash mixture to the prepared casserole dish.
3Lightly whisk eggs in a bowl; stir in sour cream, Parmesan cheese, basil, and black pepper. Pour egg mixture over squash mixture; top with mozzarella cheese.
4Bake in the preheated oven until squash is tender and mozzarella cheese is bubbling, about 30 minutes.
Footnotes
Tip
Aluminum foil helps keep food moist, ensures it cooks evenly, keeps leftovers fresh, and makes clean-up easy.