In a large wok or Dutch oven over medium-high heat, sauté the potatoes, mushrooms, leek, onion, garlic and ginger in the butter until the onions are translucent. Season with salt and pepper. Add the spices and stir to coat the vegetables. Add the tomatoes and sugar. Bring to a boil. Cover, reduce the heat and simmer gently for about 30 minutes, stirring frequently.