- 5 pounds (about 12 medium-sized) Yukon gold potatoes, peeled and quartered
- 5 cups half-and-half
- 8 cloves garlic, peeled and gently smashed
- 4 sprigs fresh thyme
- 2 bay leaves
- 10 tablespoons butter, divided
- Kosher salt and black pepper
- 5 large egg yolks and 1 egg white, divided
- 1 bunch fresh sage leaves
- Coarse salt, to top
- Preheat oven to 450°F. Butter a large baking dish and set aside.
- 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.
- 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.
- 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.
- When the butter has melted, rice the potatoes into the pot. Stir well until the potatoes are broken down and the butter is incorporated.
- Mix in 1 1/2 cups of the reserved half-and-half and 5 egg yolks. Season generously with salt and pepper.
- 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.
- 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.
- 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.
- Top the mashed potato casserole with the crispy sage and serve warm.