Lay the tomatillos, serranos, and garlic on a baking sheet lined with foil and place 4 inches below a very hot broiler. When the tomatillos blister, blacken and soften on one side, about 7 minutes, turn them over and roast the other side, 6 to 7 minutes more. Keep an eye on the garlic and serranos since they'll take less time to roast, and you'll need to remove them sooner. Scrape the tomatillos along with their juices into a blender or food processor. Add the peeled garlic, serranos, crema, and 2 teaspoons salt. Process until smooth, then scrape into a very large mixing bowl. Whisk in the chicken broth.
Lower the oven temperature to 350 degrees, if you are planning on baking the casserole right away and lightly coat a 9 x 13-inch baking dish with non-stick spray.
Heat the olive oil in a large (10-inch) skillet over medium high heat. Once the oil is hot, add the onion and saute until golden brown. Scoop in the mushrooms and continue sauteing until the mushrooms are soft and have started to brown. Remove from the heat, and scrape them into the tomatillo sauce. Fold in the turkey, pasta, and cilantro making sure that everything is well coated. Taste and adjust the salt, if necessary.
Transfer the mixture to the lightly coated 9 x 13-inch baking dish. (If you are preparing this dish ahead, allow it to cool, then cover tightly with plastic wrap. Refrigerate until about 30 minutes before baking to allow the casserole to lose its chill.)
Combine the queso anejo, panko, and smoked paprika in a small bowl. Sprinkle this crumb mixture evenly over the top of the casserole. Place on the center rack of the preheated oven and bake for about 30 to 35 minutes or until bubbly and brown. (It will take longer if you're baking the refrigerated version.)
I prefer using fresh pasta which you can find in most grocery stores. Follow the package directions, but cook it for half the time listed on the package to achieve the right consistency.