Directions

• Step 1

  Preheat oven to 350 degrees F (175 degrees C). Spray a 9x13-inch casserole dish with cooking spray.

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• Step 2

  Bring a large pot of salted water to a boil; cook squash in the boiling water until very tender, 10 to 15 minutes. Drain and transfer squash to a large bowl; mash until evenly smooth.

• Step 3

  Heat butter in a skillet over medium heat; cook and stir onion in the melted butter until tender, 5 to 10 minutes. Mix onion, cream of mushroom soup, Cheddar cheese, bread crumbs, milk, mayonnaise, sugar, eggs, salt, and pepper into mashed squash. Spread mixture evenly into the prepared casserole dish.

• Step 4

  Bake in the preheated oven until bubbling and edges are lightly browned, 45 minutes to 1 hour.