Peel potatoes and cut into 2-inch pieces. Place in a saucepan and cover with water. Bring to a boil over medium high heat, cooking until softened (10-15 minutes) Drain.
In a wok or skillet heat oil over medium high heat. Stir in mustard seeds and bay leaf. When seeds start jumping add ginger and onion. Reduce heat to medium low and add next 5 ingredients (turmeric - salt) and the potatoes. Cook stirring occasionally.
Stir in tomatoes and a little water if needed and cook a few minutes more or until potatoes are tender.