Heat the oil in a large pot over medium-high heat. Add the onion and sauté for 5 minutes, then add the ginger, garlic and potatoes and sauté for 2 minutes more. Add the water, tomato sauce and curry powder and bring to a simmer. Stir well, cover with a lid and cook 6 to 8 minutes. Add the frozen mixed vegetables and coconut milk and simmer for 5 to 10 minutes until the vegetables are tender. Season to taste with salt and black pepper.