Preheat oven to 425 degrees and place oven rack in center position

Grab two large bowls. In each bowl place 1 cup cream, 1/2 cup parmesan cheese, 1/2 teaspoon thyme the white parts and light green parts of onions (half in each bowl, reserving dark greens for later) 2-3 generous pinches of sea salt and pepper. Whisk well. Set aside

Using a mandolin or sharp knife, slice potatoes thinly. Place sweet potatoes in one bowl of cream and white in the other bowl. Toss and stir until potatoes are covered with sauce

Arrange slices in baking dish in rows (three white, two orange). I like to lean each row in the opposite direction of the previous row. Pour the cream that remains in each bowl over potatoes. Sprinkle with remaining 1/2 cup parmesan cheese and some more sea salt and pepper

Cover baking dish with lid or foil and bake for 30 minutes. Remove foil, add grated gruyere to top of potatoes. Bake another 20 minutes until potatoes are soft and cheese is crispy and golden

Garnish with desired amount of reserved greens from onions, serve