Make-Ahead Mashed Potato Casserole

Instructions

1. Preheat oven to 450°F. Butter a large baking dish and set aside.
2. Add the potatoes to a large pot or dutch oven and cover with the half-and-half. Add garlic, thyme, and bay leaves and season with salt.
3. Bring to a gentle boil, reduce to a simmer, and continue to cook until a fork can pierce the potatoes with no resistance, about 20 minutes.
4. Strain the potatoes, reserving the half-and-half for later. Discard the thyme and bay leaves. Return the pot to low heat and melt 8 tablespoons of butter in it.
5. When the butter has melted, rice the potatoes into the pot. Stir well until the potatoes are broken down and the butter is incorporated.
6. Mix in 1 1/2 cups of the reserved half-and-half and 5 egg yolks. Season generously with salt and pepper.
7. Transfer the potato mixture to the buttered baking dish and smooth with a spatula into an even layer. In a small bowl, microwave the 2 remaining tablespoons of butter (covered), to melt, allow to cool slightly, and mix in 1 egg white. Pour the butter and egg mixture over the potatoes and tilt the baking dish as necessary to cover the potatoes.
8. Using the back of a spoon, make indentations or waves in the potatoes. Transfer to the oven and bake until well-browned, about 30 minutes.
9. Meanwhile, heat a small layer of oil in a skillet over medium-high heat. Fry the sage leaves, one at a time, until crisp, about 15 seconds. Sprinkle with coarse salt.
10. Top the mashed potato casserole with the crispy sage and serve warm.