Ingredients

Filling:

3 pounds yellow squash, cut 1/8-inch thick

2 pounds zucchini, cut 1/8-inch thick

3 tablespoons unsalted butter, plus additional for greasing

1 medium yellow onion, minced

1 medium clove garlic, minced

2 large eggs, lightly beaten

8 ounces sour cream

1/3 cup mayonnaise

4 ounces sharp white Cheddar, freshly grated

1 teaspoon chopped fresh thyme

10 fresh basil leaves, chiffonade

2 teaspoons kosher salt

1/4 teaspoon freshly ground black pepper

3 tablespoons minced fresh flat leaf parsley

Topping:

32 butter crackers, crushed

2 tablespoons unsalted butter, melted

3 tablespoons grated Parmesan

Directions

1. For the filling: Heat a large high-sided skillet over medium-high heat. Add half the squash and zucchini and a pat of butter if desired. Stir, then reduce the heat to medium. Cover and let steam (you are not looking to get color on the squash, just to release its moisture), stirring occasionally, about 5 minutes. Transfer to a colander and let drain for at least 1 hour. Repeat with the second half of the squash and another pat of butter.
2. When ready to bake, preheat the oven to 350 degrees F and grease a 9-by-13-inch casserole dish with butter.
3. In the same skillet, heat the remaining tablespoon butter and cook the onions until translucent, about 5 minutes. Stir in the garlic and cook for an additional minute. Allow to cool slightly.
4. In a large bowl, combine the eggs, sour cream, mayonnaise, Cheddar, thyme, basil, salt, pepper and 2 tablespoons parsley. Stir in the squash mixture and the onion mixture. Transfer to the prepared baking dish.
5. For the topping: In a small bowl, mix the cracker crumbs with the melted butter. Sprinkle over the top of the casserole. Sprinkle over the Parmesan and bake until golden brown, about 30 minutes. Garnish with the remaining tablespoon parsley. Allow the casserole to rest for 10 to 15 minutes before serving.

Ingredients 

• ▢ ¼ cup unsalted butter
• ▢ 1 tablespoon minced garlic
• ▢ ½ cup finely chopped yellow onion
• ▢ ¼ cup all-purpose flour
• ▢ 2 cups whole milk
• ▢ ¾ teaspoon kosher salt
• ▢ ½ teaspoon black pepper
• ▢ ⅛ teaspoon nutmeg
• ▢ 2 pounds yukon gold potatoes, peeled and cut into ⅛” thick slices
• ▢ 1 cup shredded gruyere cheese
• ▢ 1 cup shredded sharp cheddar cheese
• ▢ 2 teaspoon chopped chives

Instructions 

• Preheat the Oven – Set the oven rack to the middle position. Heat to 400ºF (200ºC).
• Make the Cream – Melt butter in a large skillet over medium-low heat. Add garlic and onions, and saute until fragrant, 2 minutes. Add flour, whisk to combine, stir and cook for 2 minutes. Gradually whisk in milk and cook until thickened over medium heat, 2 to 3 minutes. Turn off the heat and whisk in salt, pepper, and nutmeg.
• Add the Cheeses – Add ½ cup gruyere and ½ cup sharp cheddar cheese, and stir to combine.
• Assemble the Casserole – Grease the bottom and sides of an 8-inch square casserole dish. Layer half of the potatoes in overlapping rows in the dish. Spread half of the cheese sauce over the potatoes. Sprinkle ¼ cup gruyere and ¼ cup cheddar over the sauce. Layer the remaining potatoes, followed by cheese sauce, and sprinkle with the remaining gruyere and cheddar cheese.
• Bake – Cover with foil and place on a baking sheet. Bake for 30 minutes. Remove the foil and continue baking until the potatoes are tender and the cheese is browned and bubbly, 20 to 25 minutes.
• To Serve – Transfer the casserole to cool on a wire rack for 10 to 15 minutes before serving. Garnish potatoes with chives.

Notes

• Make Ahead: Assemble, cover, and refrigerate for 2 days before baking. More baking time may be needed as the ingredients and pan will be cold. 
• Storing: Store in an airtight container in the refrigerator for up to 5 days. Freeze for up to 1 month. 
• Reheating: Cover and reheat a small portion in the microwave on a high setting in 15 to 30-second increments until hot. Alternatively, cover and reheat in the oven at 350ºF (177ºC) until hot.