Preheat the oven to 375 degrees F (190 degrees C).
Heat oil in a large skillet over medium heat. Cook squash and onion in hot oil, stirring occasionally, until tender, about 10 minutes.
Meanwhile, mix together eggs, 1 cup Cheddar cheese, milk, sugar, flour, salt, and pepper in a large bowl. Stir in squash mixture and melted butter; transfer to a 2-quart casserole dish. Top with remaining 2 cups Cheddar cheese.
Bake in the preheated oven until casserole is set and cheese is crisp and starting to brown, 35 to 45 minutes. Allow to stand for 15 minutes before serving.
Nutritional Facts
Per 8 servings
Calories: 388
Carbohydrate: 9g
Fat: 32g
Fiber: 1g
Protein: 17g
Sugar: 6g
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