Step 1: Preheat oven to 400°F (200°C). Place husked and rinsed tomatillos, onion wedges, unpeeled garlic cloves, and jalapeños (if using) on a baking sheet. Roast for 20-25 minutes, or until softened and lightly charred. Once cooled, peel the roasted garlic.
Step 2: While tomatillos roast, remove stems and seeds from ancho and guajillo chiles. Place them in a heat-proof bowl and cover with boiling water. Let soak for 20-30 minutes until very soft and pliable. Reserve 1/2 cup of the soaking liquid.
Step 3: Transfer roasted tomatillos, onion, peeled roasted garlic, rehydrated chiles, 1/2 cup fresh cilantro, cumin, oregano, salt, and pepper to a blender. Add a splash of the reserved chile soaking liquid as needed and blend until smooth. Taste and adjust seasoning.
Step 4: Pat chicken thighs dry and season generously with salt and pepper. In a large Dutch oven or heavy-bottomed pot, heat 1 tablespoon of olive oil over medium-high heat. Sear chicken thighs in batches until beautifully browned on all sides. Remove chicken and set aside.
Step 5: Reduce heat to medium. Add remaining 1 tablespoon olive oil, then add diced carrots, celery, and red bell pepper to the pot. Cook, stirring occasionally, for 5-7 minutes until softened. Deglaze with a splash of broth if necessary.
Step 6: Return seared chicken to the pot. Pour in the blended tomatillo sauce and remaining 4 cups chicken broth. Bring to a gentle simmer, then reduce heat to low, cover, and cook for 45-60 minutes, or until chicken is fork-tender.
Step 7: Carefully remove chicken from the pot and shred using two forks. Return shredded chicken to the stew. Stir in the fresh lime juice and remaining fresh cilantro. Taste and adjust seasoning as needed. Serve hot with desired toppings and sides.