Step 1

Preheat the oven to 375° and butter a 9-by-13-inch baking dish. Put the potatoes and garlic in a large pot and cover with water by about 1 inch; season with salt. Bring to a boil, then reduce the heat and simmer until the potatoes are fork-tender, about 20 minutes.

Step 2

Drain the potatoes and garlic well. Return to the pot off the heat and add the butter, cream cheese, half-and-half, chili powder, 1 1/2 teaspoons salt, cayenne and half of the pepper jack. Mash with a potato masher until smooth. Season with more salt and cayenne to taste.

Step 3

Spread the potatoes in the baking dish. Top with the remaining pepper jack and sprinkle with chili powder. Bake until the cheese is melted and the potatoes are lightly browned around the edges, 25 to 30 minutes. Sprinkle with scallions before serving.