Step 1Peel the baking and sweet potatoes and slice them 1/8 inch thick.
Step 2Combine the eggs, whipping cream, garlic and shallot and blend well.
Step 3Heat the oven to 300 degrees. In a 13x9-inch baking dish, arrange 1 layer of sliced baking potatoes, then sprinkle them with some of the cheese and season to taste with salt and pepper. Top them with a layer of sweet potatoes, then more cheese, salt and pepper. Keep alternating layers until all potatoes are used, ending with a layer of cheese. Pour the cream mixture over the top and cover the pan with foil. Bake for 2 hours.
Step 4Remove the foil and bake until the potatoes are golden brown, another 40 minutes.