Heat oil over medium heat in a dutch oven large enough to hold the chicken. Cook onion until golden, about 5 minutes, then add garlic, jalapeno, and cilantro and cook until fragrant, about a minute. Stir in fish sauce, tamarind, water, and brown sugar and bring to a boil. Reduce heat and simmer 5 minutes, until slightly thickened. Add chicken and cook, stirring frequently, until no longer pink inside, about 12 minutes.