Dice and peel potatoes into I inch cubes and place in large sauce pan and boil for 10 mins. In large cooking pot add vegetable oil on medium heat with lemongrass, ginger, cayenne pepper and red curry paste and cook for 1 min. Add chicken stock and stir together. Add fish sauce, brown sugar and potatoes and simmer for 15 mins. Add coconut milk and mushrooms and stir for 10 mins. Add lime juice and garnish with fresh cilantro and serve.