1.5 lbs sweet potatoes
1.5 lbs Yukon gold potatoes
2 shallots, diced
1/4 cup butter, divided
2 cups heavy cream
2 tbs chopped parsley
1 tbs chopped chives
1 tbs kosher salt
1/2 tbs ground white pepper
1/8 tbs freshly grated nutmeg
2 cups milk
1.5 cups shredded Gruyere cheese
Preheat oven to 375 degrees
Saute the shallots in 3 tbs melted butter in a saucepan over medium heat for two minutes. Stir in cream and parsley, chives, salt, white pepper, and nutmeg. Cook for two minutes and let cool for 15 minutes.
While cooking, peel and thinly slice all potatoes. Combine sliced potatoes and milk in a large, microwave safe bowl. Cover bowl with plastic wrap and microwave at high for five minutes. Take out, stir, and repeat for another five minutes. Remove and drain mixture.
Layer one-third of the sliced Yukon gold potatoes in a well-greased (with butter) baking dish. Top with one-third of the sweet potatoes.
Spoon one-third of cream mixture over these two layers and sprinkle with 1/2 cup of the Gruyere cheese. Repeat this for the remaining four layers and top with Parmesan cheese.
Cut remaining butter in to small pieces and dot over the top of the dish. Cover with foil and bake for 30 minutes at 375. Uncover and bake another 20 minutes, then let stand for 10 minute. Serve.