For stock: Season chicken thighs well with salt and black pepper. In a large Dutch oven, heat 1 tablespoon olive oil over medium. Sear thighs skin sides down in hot oil until browned, about 6 minutes. Turn thighs and cook 4 minutes more. Transfer to a plate. Add the white ends of green onions (save the green tops for garnish), the cilantro stems (save the leaves for garnish), smashed garlic, coriander seeds and cumin seeds to pot; cook and stir until aromatic, 30 to 60 seconds. Add celery, carrots, coarsely chopped onion, and chopped jalapeño pepper. Return chicken to pot. Add 2 quarts water and 1 tablespoon salt. Bring to a boil, reduce heat, cover and simmer 2 hours. Remove chicken and set aside to cool. Strain stock and skim fat.