Preheat the oven to 350°F and place the oven rack in the middle position. Grease a 9x13-inch baking dish with butter. Set aside.

In a skillet, melt 1 tablespoon butter over medium heat. Add sliced onions and cook for 5 minutes until they are soft and translucent. Add the mushrooms and sauté until they release moisture and turn golden, about 7 minutes more. Remove from heat and set aside.

Peel and slice the potatoes into ⅜ inch (9 mm) thick matchsticks. A mandoline slicer is a handy tool for this.

Layer ⅓ of the potatoes in the bottom of the prepared baking dish. Spread half the onion-mushroom mixture evenly over the potatoes. Repeat the layers, finishing with a top layer of potatoes.

In a bowl or glass measuring cup, whisk together the heavy cream, Dijon mustard, white pepper, and salt. Pour the mixture over the layers of potatoes, onions and mushrooms. It should come up ¾ of the way in the dish. If yours appears to be less than that, add water, a tablespoon at a time, until the liquid comes ¾ of the way up the dish. Press down on the top layer of potatoes with a rubber spatula so that they are moist from the liquid (they will not be completely submerged).

Sprinkle the top with breadcrumbs and dot with small cubes of the remaining 2 tablespoons of butter. Cover tightly with foil and bake for 60 minutes.

Remove the foil and scatter chopped capers over the top of the dish. Bake uncovered for an additional 20-25 minutes or until the top is golden and the potatoes are tender. Let the casserole rest for 10-15 minutes before serving.