2 shallots, diced
1/4 cup butter, divided
2 cups heavy cream
2 tablespoons chopped parsley
1 tablespoon chopped chives
1 teaspoon kosher salt
1/2 teaspoon ground white pepper
1/8 teaspoon freshly grated nutmeg
1 1/2 pounds Yukon gold potatoes
1 1/2 pounds sweet potatoes
2 cups milk
1 1/2 cups (6 oz.) shredded Gruyère cheese
1/4 cup grated Parmesan cheese
1. Preheat oven to 375°. Sauté shallots in 3 Tbsp. melted butter in a saucepan over medium heat 2 minutes. Stir in cream and next 5 ingredients; cook 2 minutes. Remove from heat; cool 15 minutes.
2. Meanwhile, peel and thinly slice all potatoes. Combine sliced potatoes and milk in a large, microwave-safe bowl. Cover with plastic wrap, and microwave at HIGH 5 minutes. Uncover and gently stir mixture. Re-cover and microwave 5 more minutes. Drain mixture, discarding milk.
3. Layer one-third of Yukon gold potatoes in a well-greased (with butter) 9- x 13-inch baking dish; top with one-third of sweet potatoes. Spoon one-third of cream mixture over potatoes, and sprinkle with 1/2 cup Gruyère cheese. Repeat layers twice, and top with Parmesan cheese. Cut remaining 1 Tbsp. butter into small pieces, and dot over top. Cover with foil.
4. Bake at 375° for 30 minutes. Uncover; bake 20 minutes or until browned. Let stand 10 minutes.