Scalloped Potato and Sweet Potato Gratin

1-1/2 lbs. potatoes, preferably Yukon gold
1-1/2 lbs. sweet potatoes
2 c. heavy whipping cream
1/4 c. butter
2 garlic cloves, minced
1 Tbsp. minced Italian parsley
1 Tbsp. minced fresh rosemary
1 Tbsp. minced fresh sage
1 Tbsp. minced fresh thyme
1-1/2 tsp. salt
3/4 tsp. pepper
1-1/4 c. Gruyere cheese, grated and packed (roughly 5 oz)

Fill a large bowl with cold water. Slice potatoes and sweet potatoes 1/8" thick. Place in water bowl to keep from browning. Combine cream, butter and garlic in medium saucepan and bring to a simmer. Remove from heat. Mix all herbs in a small bowl and mix salt and pepper in separate small bowl.

Butter 9X13 glass baking dish. Drain potatoes and pat dry. Transfer half the potatoes and sweet potatoes to baking dish; distribute and spread evenly. Sprinkle with half salt-pepper mixture, herb mixture and cheese. Repeat with remaining potatoes, salt-pepper mixture, herbs and cheese. Pour cream mixture over gratin, pressing lightly to submerge potatoes as much as possible. Can be made up to six hours ahead. If made ahead, cover and refrigerate.

Preheat over to 400 degrees. Cover gratin tightly with foil. Bake 30 minutes. Uncover and bake until top of gratin is golden and most of the liquid is absorbed 22-25 minutes. Let stand 10 minutes before serving.