Souffléed Sweet Potatoes

Preheat an oven to 375°F.

In a bowl, stir together the sweet potatoes, garlic cloves and olive oil, and season with salt and pepper. Transfer to a baking sheet. Bake, stirring occasionally, until the sweet potatoes and garlic cloves are tender, about 45 minutes. Let cool for 15 minutes. Peel the garlic cloves and place in a food processor. Add the sweet potatoes and puree until smooth, about 30 seconds.

Reserve 1/2 cup of the Gruyère in a small bowl. In another bowl, stir together the remaining Gruyère and the Parmigiano-Reggiano.

In a 2 3/4-quart round Dutch oven over medium heat, melt the butter. Add the flour and cook, stirring constantly, until pale golden, about 2 minutes. Remove the pot from the heat and slowly whisk in the milk. Return the pot to medium heat and cook, stirring constantly, until the mixture is very thick, 2 to 3 minutes. Reduce the heat to low and stir in the cheese mixture in several additions, allowing each addition to melt completely before adding more. Remove the pot from the heat and stir in the sweet potato mixture, then the egg yolks.

In a bowl, whisk the eggs whites to medium peaks. Carefully fold the whites into the sweet potato mixture in 3 additions. Transfer the pot to the oven and bake, uncovered, until the soufflé puffs up to the rim of the dish, the center is set and the top is lightly browned, about 45 minutes; during the last 10 minutes of baking, sprinkle the top of the soufflé with the remaining Gruyère.

Remove the pot from the oven, sprinkle the soufflé with the parsley and chives, and serve immediately. Serves 8.