Step 1

Preheat oven to 350 degrees. In a large pot, bring potatoes to a boil in salted water over high. Reduce heat to a rapid simmer and cook until potatoes are tender when pierced with a knife, about 12 minutes. Drain, return to pot, and mash. Add 6 tablespoons (3/4 stick) butter and stir until melted. Stir in half-and-half and sour cream and season with salt and pepper. Transfer to a shallow 2-quart baking dish.

Step 2

In a medium bowl, stir together 2 tablespoons melted butter and mustard; add breadcrumbs and stir until coated. Sprinkle over potatoes and bake until breadcrumbs are golden brown, about 20 minutes.