Preheat the oven to 400 degrees. In a large bowl, toss the squash with the oil and season with salt. Spread the squash on a nonstick baking sheet and roast, turning once, for about 25 minutes, until tender and lightly browned in spots. Transfer the squash to a food processor and puree until smooth.

Meanwhile, in a large pot, cover the potatoes and garlic with cold water and bring to a boil. Simmer over moderate heat until tender, about 20 minutes.

Drain the potatoes and garlic in a colander, shaking out the excess water. Add the half-and-half and butter to the pot and heat until melted. Remove from the heat. Press the potatoes and garlic through a ricer into the pot and season with salt. Stir in the butternut squash puree and cook over moderate heat until very hot. Transfer the mashed potatoes to a bowl and serve right away.

The dish can be made earlier in the day and kept at room temperature; warm over moderate heat, stirring constantly.