How to Make It
Preheat the oven to 375°. Butter and flour a 2-quart soufflé dish. In a medium saucepan, cover the potatoes with cold water and bring to a boil. Simmer until tender, about 20 minutes.
Drain the potatoes and return them to the saucepan. Shake the pan over high heat to dry out the potatoes, about 1 minute. Add the 2 tablespoons of butter and mash the potatoes. Stir in the half-and-half and let the potatoes cool slightly. Stir in the cheese and the herbs and season with salt and pepper. Stir in the 5 egg yolks. Transfer the potato mixture to a large bowl.
In another large bowl, using an electric mixer, beat the 7 egg whites until foamy. Add the cream of tartar and beat until stiff peaks form. Stir one-third of the beaten whites into the potato mixture. Using a rubber spatula, fold in the remaining whites until no streaks of white remain. Scrape the mixture into the prepared soufflé dish and bake in the bottom third of the oven for about 45 minutes, until the soufflé is puffed and richly browned. Serve immediately.