In a large skillet over high heat, melt the two tablespoons of butter in one tablespoon of the olive oil. Once hot and frothy, add mushrooms and cook over high heat until browned, about five minutes. Add cognac off the heat and then return the pan to the heat to evaporate the alcohol. The mixture may flame, that is fine.

Remove to a plate to cool and lay the mushrooms flat. Stacking them in a bowl will make them give up water.

Heat the pan back to medium high and add the remaining tablespoon of oil. Once hot add the pancetta.

Cook the pancetta to crisp then add the onions. Sauté for three minutes and add the potatoes and milk.

Cover and simmer for 8 minutes,* stirring every 3-4 minutes or until cooked through. Remove from heat and stir in the Boursin cheese.

*While potatoes are cooking, make topping by melting the two tablespoons of butter in a small sauce pan over medium high heat and add the panko bread crumbs.  Toss and stir for a few minutes to brown then remove from heat and set aside.

Preheat oven to 350 degrees F.

To assemble, pour potato mixture into a 9x9x2½-inch casserole dish. Top with the reserved mushrooms then top with the shredded cheddar and the toasted bread crumbs.

Bake for 20 minutes with a piece of foil on the rack below the dish to catch any drips.

When the au gratin is right out of the oven, it will be loose and as it sets, it will firm up.