1. Preheat the oven to 350. Grease a 9 x 13 glass baking dish.
2. Peel and thinly slice the potatoes. Thinly slice the mushrooms.
3. Cover the bottom of the baking dish with 1/2 of potatoes. Place all the mushrooms on the top and cover with another 1/2 of potatoes. Season with salt and pepper. Pour the vegetable stock over all to a depth of 1/2 .
4. Oil one side of a sheet of aluminum foil and lightly cover the potatoes, oiled side down. Bake for 1 hour, then remove the foil. Bake another 30 - 45 minutes, or until a knife goes through the center easidly. Remove from the oven. Place a heavy object on top of the terrine to compress it while cooling, for about 1 hour.
5. To serve: Cut the terrine into squares, place on a greased cookie sheet and reheat at 350 for 10 - 15 minutes.