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11 lbs. mixed zucchini and yellow squash, sliced into 1/8-inch rounds
3 lbs. white onion, peeled and thinly sliced
2 garlic cloves, minced
2 tsp. sugar
6 short thyme stems
2 fresh bay leaves
1/4-inch strip of lemon peel with the pith removed
3 cups water
5 egg yolks
1/2 cup mayonnaise, Duke’s preferred
11/2 cups grated Gruyère cheese, divided
Fresh ground pepper
1 Tbs. lemon zest
1/4 cup chopped flat-leaf parsley, divided
23/4 cups dried bread crumbs or panko, divided
1 Tbs. unsalted butter at room temperature
1/4 cup unsalted butter, melted
Preheat oven to 350°F. Line two baking sheets with parchment paper.
Place the squash in a large mixing bowl, sprinkle lightly with salt, and toss gently. Arrange the slices in a single layer on the prepared baking sheets and bake for 15 to 20 minutes, until just dry but not browned. (Work in batches if necessary and change parchment paper if it gets wet.) Cool to room temperature. Set aside two cups of the sliced squash to use to gussy up the top of your casserole before baking.
Place the onions in a pot. Add the garlic, one tablespoon of salt, sugar, thyme, bay leaves, and lemon peel. Add the water and bring to a boil over high heat. Reduce heat to low and simmer until all the water has evaporated, but the onions have not browned, about five minutes. Discard the lemon peel, thyme, and bay leaves. Spread the onions out on a parchment-lined baking sheet and cool to room temperature.
In a mixing bowl, combine the eggs, yolks, mayonnaise, and 1 1/4 cups of the cheese. Add the onions and season to taste with salt, pepper, and lemon zest. Add all but two tablespoons of the parsley. Combine the squash, onion mixture, and two cups of the bread crumbs in a large bowl and mix well.
Set a rack in the center of the oven and preheat to 425°F.
Grease a 12- by seven- by 1 3/4-inch baking dish with the room temperature butter and add the squash mixture. Arrange the reserved slices of squash around the top of your casserole to make an attractive presentation.
In a mixing bowl, combine the remaining bread crumbs, parsley, and cheese with the melted butter. Season to taste with salt and pepper, mix well, and sprinkle on top of the casserole.
Bake for 15 minutes, or until hot through. (If it needs more time, bake it longer—use your best judgment.) Serve immediately.