Step 1

Heat the oven to 400°F.

Step 2

Use 2 tablespoons of the soft butter and coat the inside of a 9x13-inch oven-proof dish.  Sprinkle the Parmesan over the butter turning to cover the bottom and sides.  Set aside.

Step 3

Combine the cream, sage, rosemary, thyme, garlic, and nutmeg in a saucepan and bring to a simmer over medium-low heat.  Do not allow to boil.

Step 4

In a small bowl, whisk the eggs together with 2-3 tablespoons of the cream mixture,  then slowly add the mixture to the hot cream while continuing to whisk.

Step 5

Add the remaining 2 tablespoons of soft butter and the 2 tablespoons of flour to a small dish.  With the back of a tablespoon mash the butter and flour together until blended into a paste.

Step 6

Whisk the butter-flour paste into the hot cream mixture and simmer for 1 minute until lightly thickened. Set the mixture aside.

Step 7

With a sharp knife or a mandoline, slice the peeled sweet potatoes into 1/8-inch thick rounds.

Step 8

Layer 1/2 of the sweet potato rounds on the bottom of the prepared baking dish, overlapping as you go.  Sprinkle the potatoes with salt and pepper then pour 1/3 of the creamy mixture over the potatoes.  Top with 1/2 of the Gruyère.

Step 9

Repeat with another layer of potatoes, salt/pepper and 1/3 of the cream and the remaining Gruyère cheese.

Step 10

Add the final layer of potatoes, salt/pepper and the last of the cream.  With a spatula, press down on the potatoes to compress them.

Step 11

Tear off a sheet of foil and coat one side with non-stick oil spray.  Cover the dish of potatoes with the foil and transfer the dish to to the oven.  Bake  the potatoes for 45 minutes.

Step 12

Remove the foil and continue baking until the potatoes are tender, browned and bubbly at the edges and no resistance when pierced with the tip of a paring knife, about 25-30 minutes.

Step 13

Transfer from the oven and serve piping hot.