In medium pot or skillet, add oil, mustard seed, cumin and turmeric. Stirring constantly. when seeds start to pop, add onion and jalapeno. Saute. Add potatoes, salt and water. simmer covered 10 minutes or until potatoes are tender. add water as necessary. add lime juice to deglaze pan. stir in peas and cilantro. heat through. spoon into warm tortillas and rice. Add sour cream if desired.