Step 1

Heat oven to 350°F. Butter an 8-inch baking dish, round or square.

Step 2

In a medium pot, cover squash with cold water, place a lid on top and bring to a boil over high heat. When boiling, reduce heat so that the water just simmers and cook squash for about 15 min. or until tender. Rinse under cold water; drain. (Do-Ahead: at this point the squash can be stored for up to two days until you are ready to assemble the gratin.)

Step 3

Heat olive oil in a non-stick pan and add onions. Sauté on medium heat, stirring often, until softened. Set aside to cool slightly.

Step 4

In a medium bowl, beat egg, salt and mayonnaise together. Add 1/3 cup of Parmesan, panko, sautéed onion and squash and fold together gently to combine. Season with pepper and more salt, if needed.

Step 5

Spoon in to prepared baking dish and top with remaining 1/3 cup of cheese. Bake 1 hour or until heated through and cheese is slightly browned. Serve hot.