Soak the parchment. Cut four 12-inch squares of parchment paper, loosely roll them together, submerge them in a large bowl of tap water and weight with a plate to keep them submerged. Soak about ½ hour. Cut four 8-inch pieces of kitchen twine for tying the packages.
Prepare the mixiotes. Turn the oven on to 325 degrees. In a large bowl, combine the mushrooms with the oil, herbs, garlic, bacon (if using), potatoes, chiles and salt. Remove the parchment from the water and lay them out onto your work surface. Scoop a portion of the mushroom mixture into the center of each one. Gather the corners to form a pouch, pinching the parchment together just above the filling and tying securely with the twine. Set on a baking sheet and bake for about 45 minutes, until the mixture is bubbling vigorously in the packages.
Serve. Open the packages in front of your guests so they can enjoy the explosion of aroma into the room. Set them out with the warm tortillas, salsa, limes and fresh cheese for everyone make tacos al gusto.
Red Chile Mushroom Mixiotes: Reduce the garlic to 4 cloves and stir in 1/3 cup Red Chile Adobo Quick Red Chile Adobo Marinade.
Cactus and Mushroom Mixiotes: Replace half the potatoes with 2 medium (5 ounces total) cooked cactus paddles; these are also delicious with the addition of Red Chile Adobo (as described above). Clean the cactus paddles: hold with a pair of tongs while trimming off the edge that outlines the paddle, including the blunt end where it was severed from the plant. Then slice or scrape off the spiny nodes from both sides. Cut into ½-inch pieces. Scoop the cactus into a medium (2- to 3-quart) saucepan and add 1 tablespoon oil and a big pinch of salt. Cover and set over medium heat and cook, stirring once or twice for about 5 minutes. Uncover the pan and continue to cook, stirring regularly until all of the liquid has evaporated and the mixture is just beginning to sizzle in the oil, about 5 minutes more. Spread onto a large plate to cool.