1 1/4cups (3ounces) thinly sliced chanterelle or porcini mushrooms, if available
1 large shallot, finely chopped
1tablespoon finely chopped chives
salt and freshly ground white pepper
In a large sauté pan, melt the butter over medium-high heat. When the foam has subsided and the butter is just beginning to turn brown, add the white mushrooms and sauté, stirring, for 1 minute; then add the shiitakes and stir for another minute; finally, add the oysters and chanterelles, if using. Cook the mushrooms until all their liquid has evaporated, about 5 to 6 minutes more. Add the shallot and the chives, stir to mix well, season to taste with salt and white pepper, and remove from the heat.
On a baking sheet, make 4 individual lasagnas by layering first a single sheet of potato, then a nice thick layer of the mushroom mixture, then another sheet of potato, and so on. Use 3 layers of mushroom mixture and end with a potato layer. Place the other baking sheet on top of the lasagnas with a weight on it and allow the lasagnas to compress for 1 hour; otherwise, they will tend to fall apart.