Step 1

Arrange racks in upper and lower thirds of oven; preheat to 350°. Prick potato skins with a fork and place cut side down on 2 foil-lined rimmed baking sheets. Roast potatoes, rotating sheets halfway through, until very tender when squeezed, 70–90 minutes. Let cool on sheets. Increase oven temperature to 400°.

Step 2

While potatoes roast, cook rosemary, sage, and 6 Tbsp. butter in a small saucepan over medium heat, stirring occasionally, until butter foams, then browns, and herbs are golden and crisp, about 5 minutes; let cool.

Step 3

Scoop out flesh from potatoes and transfer to a large bowl (discard skins). Spoon about half of warm butter and fried herbs into bowl with potatoes. Add breadcrumbs to saucepan with remaining butter-herb mixture and toss to coat; season with salt and pepper. Transfer to a small bowl.

Step 4

Heat remaining 2 Tbsp. butter in same saucepan over medium. Add shallots and cook, stirring often, until translucent and golden around the edges, about 5 minutes. Add cream and bring to a simmer. Remove from heat and, using an immersion blender, blend cream mixture until smooth. Transfer to bowl with potatoes. Add eggs and Parmesan. Blend with immersion blender until light and smooth. Season with salt and pepper.

Step 5

Transfer potato mixture to a shallow 2-qt. baking dish, smoothing top. Using a spatula or the back of a large spoon, create some peaks and valleys. Top with breadcrumb mixture. Bake casserole until top is golden and crisp, 20–25 minutes. Sprinkle lemon zest over. Let cool 10 minutes before serving.

Step 6

Do Ahead: Casserole, without breadcrumbs, can be made 3 hours ahead. Fully assembled casserole can be made 1 day ahead; cover with foil and chill. Let come to room temperature before baking.