In a medium casserole or Dutch oven, heat the oil. Season the pork with salt and pepper and cook over high heat until browned on 2 sides, about 2 minutes per side. Add the celery and onion and cook over moderate heat, stirring occasionally, until softened, about 7 minutes. Add the diced chile, garlic, chile powder, cumin and oregano and cook, stirring frequently, until fragrant, about 3 minutes. Add the chicken stock and bring to a boil. Add the carrots, potatoes, tomatoes and tomatillos, cover and simmer over low heat until the pork is cooked through, about 25 minutes.