- Put the potatoes in a large stockpot, add water to cover by 2 inches and generously salt the water. Bring to a boil over medium-high heat, reduce the heat to medium-low and cook until the potatoes are tender when pierced, 20 to 30 minutes. Drain well in a colander and let stand for 5 minutes. Set a potato ricer over a large bowl and pass the potatoes through in batches.
- Preheat an oven to 450°F. Butter a 2-quart baking dish.
- In a small saucepan over medium heat, warm the half-and-half and butter until the butter melts and small bubbles appear along the edges of the pan. Remove from the heat.
- Add the half-and-half mixture to the potatoes and stir until just combined. Add 1 cup of the cheese and 3 tablespoons of the chives and stir until just combined. Season with salt and pepper. Spoon the potatoes into the prepared baking dish and spread evenly. Sprinkle with the remaining 1 cup cheese.
- Bake until the cheese is melted and lightly browned, 10 to 15 minutes. Let stand for 10 minutes, then sprinkle with the remaining 1 tablespoon chives. Serve immediately.
- TIP: Peel and quarter the potatoes 1 day in advance; place them in a bowl, add cold water to cover and refrigerate. If desired, cook and mash the potatoes up to 2 hours before serving, but do not add the cheese and chives. Cover the potatoes tightly with plastic wrap and keep warm in a bain-marie (hot water bath). Just before serving, fold in the cheese and chives and bake as directed.