This sweet potato gratin has everything you need to make a crowd-pleasing dish: a garlicky herb cream sauce, gruyere and parmesan cheese, and thinly sliced sweet potatoes.
Prepare your sweet potatoes by rinsing and peeling.
Slice your sweet potatoes into 1/8 inch rounds using a mandoline or a sharp knife. You can prepare the sweet potatoes ahead of time as this can be time consuming.
Preheat the oven to 400 degree F and butter two pie dishes. Also butter foil for covering the pan later. Sprinkle parmesan over the bottom of your dish(es).
Combine cream, sage, rosemary, garlic, nutmeg and a pinch of salt in a pan over medium heat. Whisk often until the garlicky cream sauce begins to simmer. Keep stirring for about 10 minutes, until the sauce thickens somewhat.
In a large bowl crack the eggs and beat them just enough to break up the yolks. Slowly add the cream sauce to the eggs, whisking as you combine. Put your cream sauce, sweet potato slices, and the grated cheese near your baking dishes.
Divide 1/3 of the sweet potatoes between the two pie dishes. In one dish assemble the sweet potato slices into a layer over the grated parmesan, each piece somewhat overlapping the last starting on the outside border and moving in towards the center of the dish. Repeat with second pie dish. Sprinkle both with salt and pepper to taste and divide 1/3 of the cream sauce between the two pie dishes. Top with 1/4 C Gruyere cheese on each pie dish.
Repeat this step with another layer. Add 1/3 of the potatoes between the two dishes, sprinkle with salt and pepper, divide 1/3 of the cream sauce and sprinkle 1/4 C Gruyere cheese on each dish. For the final layer divide the remaining 1/3 of potatoes, salt and pepper, the rest of the egg mixture, but set the remainder of the Gruyere cheese (about 2/3 cup) to the side. Press down firmly with a spatula to compact the potato mixture. Cover with foil, place in oven and bake for 40 minutes, or until the potatoes are soft enough to easily pierce with a fork. Remove the foil, sprinkle on the rest of the Gruyere cheese and little Parmesan (if you have extra) and cook for an addition 25 to 30 minutes until bubbly and browning.
Let cool for about ten minutes and then serve.