Cartoccio is Italian for "paper bag," a culinary technique similar to the French method of cooking something "en papillote." Baking the dish in a bag concentrates the flavors, keeps the food moist and tender, and leads to an exciting presentation. This can be served on its own, over pasta, as an antipasto, or many other ways.
Preheat oven to 500 degrees F for 30 minutes (425 degrees F for convection ovens). Lay a large piece of parchment paper atop a large piece of aluminum foil. (The dimensions should be about 24" long by 16" wide.) Spread the softened butter on the parchment, making sure to leave a 1½-inch border, and set aside. Combine the spring onions, thyme, garlic, bay leaves, and mushrooms in a bowl. Add the olive oil, a pinch of lemon zest, and toss well. Season with the salt and toss again.
Spread the mushroom mixture out evenly over one side of the parchment-foil. Fold the other half of the foil and parchment over the mixture. (You may need to remove some excess mixture if it is too crowded and make a second cartoccio.) Starting on one end, crimp the edges closed, folding over in pleats. Leave a 2-inch gap on the other end and add the stock through that hole. Make sure the stock is poured evenly so that you will be able to seal the cartoccio. Seal the cartoccio by folding/crimping; if it is not sealed properly, it won't expand. It should look like a half moon, with the foil on top and the parchment in the center enclosing the mushrooms. Place the cartoccio on a flat baking sheet or in a gratin dish and place it in the oven for 15 minutes if using a convection oven, 20 minutes if using a conventional oven. The cartoccio should expand and look like a small Zeppelin.
Remove the bag from the oven and cut it open using kitchen scissors or chef's knife (be careful to avoid being burned by the hot steam). Finish with a drizzle of olive oil, a pinch of salt, and lemon juice to taste. Plate the mushrooms, pour the jus on top, and serve alongside bread, pasta, fish, or on its own.