Preheat oven to 325 degrees. Oil a 8 1/2″ x 11″ glass or metal pan with olive oil. Set aside.
Peel the sweet potatoes and slice them thinly, as you would for scalloped potatoes. When all of the sweet potatoes have been sliced, organize the other ingredients to make it easy to assemble the casserole.
Layer 1/3 of the sweet potato slices (slightly overlapping each other) in the pan. Sprinkle them with salt and pepper, then top with 1/3 of the onion, garlic, cheese, and fresh herbs.
Repeat the layers two more times, using up all of the ingredients. Pour 1 1/2 cups heavy cream over the top of the casserole and wrap tightly with tin foil.
Bake for 45 to 60 minutes, until sweet potatoes are cooked through. Remove foil and test for doneness by inserting a knife into the center of the casserole. The sweet potatoes should be soft. If not cooked completely, add the remaining 1/2 cup of cream, cover again with foil and continue baking for 15 minutes more.
Remove the foil and continue cooking for 15 to 20 minutes so the extra liquid is absorbed and the cheese browns slightly. Let the casserole rest, covered for 10 minutes before serving. Cut into squares or serve with a spoon.