Cook potatoes in salted water until tender. Drain and mash.
Preheat oven to 350°F Butter a 6 cup souffle dish.
Cook the butter and onion over medium heat until soft (4 - 5 minutes), add to the mashed potatoes. Stir in the egg yolks, and creams. Season with salt and pepper. Add herbs and Parmesan cheese, and stir until smooth.
Beat the egg whites until stiff, glossy peaks form. Stir 2 tbsp of the egg whites into the potato mixture to loosen it, then fold in the rest of the egg whites.
Pour the mixture into the buttered souffle dish, and bake until golden brown - 20 to 25 minutes. Start checking after 15 minutes.