For a slightly spicy version, substitute pepper Jack cheese for the cheddar. Slicing the potatoes, rather than cutting them into chunks, ensures that they will cook more evenly. For the smoothest texture, err on the side of overcooking the potatoes rather than undercooking them.

For the Topping: Adjust an oven rack to the middle position and heat the oven to 300 degrees. Pulse the bread into crumbs in a food processor, about 6 pulses. Toss the crumbs with the butter, and spread them out over a rimmed baking sheet. Bake, stirring occasionally, until golden and dry, about 20 minutes. Let the crumbs cool, then toss with the parsley and season with salt and pepper to taste; set aside.

For the Casserole: Meanwhile, place the potatoes in a large Dutch oven and cover by 1 inch of water. Bring to a boil over high heat, then reduce to a simmer and cook until the potatoes are tender (a paring knife can be slipped into and out of the center of the potatoes with very little resistance), about 15 minutes.

Drain the potatoes in a colander. Set a food mill (or ricer) over the now empty, but still warm, saucepan. Working in batches, drop the potatoes into the hopper of the food mill and process the potatoes into the saucepan. Stir in the milk, sour cream, butter, Dijon, garlic, and cayenne until uniform. Season with salt and pepper to taste.

Spread the potato mixture into a 13 by 9-inch baking dish. Sprinkle the cheddar cheese evenly over the top, followed by the bread crumbs.

To Store: Cover the dish tightly with plastic wrap and refrigerate for up to 2 days.

To Serve: Adjust an oven rack to the middle position and heat the oven to 350 degrees. Remove the plastic wrap and cover the dish tightly with foil. Bake for 20 minutes. Remove the foil and continue to bake until the casserole is heated through and the crumbs are crisp, 20 to 30 minutes longer.

To Serve Right AwayBake the casserole in a 350-degree oven, uncovered, until hot throughout, 25 to 30 minutes.