Preheat the oven to 400 degrees with the rack in the upper half of the oven.
Thinly slice the potatoes about 1/8 inch on a mandolin, food processor, or the old fashioned way, a sharp knife .
Melt the butter in a large sauté pan over high heat and add the shallots and sliced mushrooms and cook for about 6 to 8 minutes.
Add the re-hydrated porcinis to the pan (if using) and sauté until the liquid from the mushrooms has evaporated.
Season the mushrooms with salt and pepper.
Add the chicken stock, 1 1/4 cup of the cream, and chopped herbs, heat through, do not boil.
Use an oiled 3-quart baking dish.
Begin with a layer of about 1/3 of the potatoes, 1/2 of the mushrooms and cream and a little of each cheese, 1/3 of the potatoes, the remaining 1/2 of the mushrooms with cream, then the final layer of potatoes.
Pour the remaining 1/2 cup cream over the potato layers and top with the remainder of the cheeses.
Cover with foil.
Bake until the potatoes are tender and the cream is almost completely set and thickened, about 1 ½ hours.
Uncover and top with breadcrumbs and cook until golden, about 15 minutes.