Preheat oven to 425°F. Toss together tomatillos, garlic, jalapeños, poblano, white onion, and salt on a large rimmed baking sheet.
Roast until tomatillos are very soft, about 30 minutes. Transfer tomatillo mixture to a blender; add stock and cilantro. Secure lid on blender, and remove center piece to allow steam to escape. Place a clean towel over opening; process until smooth with flecks of tomatillo seeds and cilantro still visible, about 20 seconds. Reduce oven temperature to 375°F.
Meanwhile, pour oil to a depth of ¼ inch into a medium skillet over medium. Working with 1 tortilla at a time, fry until lightly crisp around edges but still pliable, 10 to 15 seconds per side. Transfer tortillas to a paper towel–lined plate to drain.
Spread 1 cup salsa verde in bottom of a 13- x 9-inch baking dish. Toss together chicken, cumin, and 1 cup salsa verde in a large bowl until well combined. Divide chicken mixture evenly among tortillas (about ¼ cup each). Roll up tortillas; place seam side down in baking dish. Spoon remaining salsa verde evenly over tortillas.
Cover baking dish with aluminum foil, and bake at 375°F until enchiladas are heated through, about 20 minutes. Sprinkle with queso fresco. Garnish with sliced red onion and crema, if desired.
