Winter Squash and Short-Rib Enchiladas

Toast coriander seeds in a small skillet over medium heat, swirling pan often and adding cumin seeds during the last 30 seconds of cooking, until fragrant, about 2 minutes. Let cool, then finely grind in spice mill or with mortar and pestle. Bring guajillo, ancho, and morita chiles and 4 cups stock to a boil in a medium saucepan over medium-high heat. Cover, remove from heat, and let sit 30 minutes to let chiles soften. Transfer chile mixture to a blender and add toasted spices, garlic, tomato paste, and oregano and purée until smooth, about 2 minutes. Preheat oven to 250 °F. Place squash, cut side down, in a 13x9" baking dish and bake until very tender, 2 –2 1/2 hours. Let cool slightly, then remove skin and discard. Meanwhile, heat oil in a medium heavy pot over medium-high. Season beef with salt and cook, turning occasionally, until browned on all sides, 10 –12 minutes. Pour off excess oil and carefully add chile purée (it may bubble vigorously) and bay leaves. Bring to a boil, cover, and transfer to oven, next to squash. Braise beef until it shreds easily, 2 –2 1/2 hours; season with salt. Skim excess fat from chile sauce and transfer meat, leaving bones behind, to a baking sheet. Remove bones and bay leaves from sauce. Let beef cool slightly, then shred with 2 forks, discarding any cartilage or silver skin. Blend sauce, 2 cups roasted squash, and remaining 1/2 cup stock in a blender until smooth, about 30 seconds; season with salt. Mix 1/2 cup squash-chile sauce into shredded beef and season with salt. Set remaining sauce aside.