Combine chilies, ginger, cumin, and black mustard seeds. Heat oil in a medium saucepan over moderately high heat. Pour in seasonings and fry until mustard seeds turn grey and start to splutter and pop. Add potatoes and cauliflower and stir-fry for 4-5 minutes, or until vegetables pick up a few brown spots.

Add tomatoes, turmeric, coriander, garam masala, brown sugar, salt, and half the cilantro. Stir well, cover, and gently cook over low heat for another 15-20 minutes until the vegetables are tender. You may want to sprinkle in a little bit of water if the vegetables start to stick to the bottom of the pan. Stirring occasionally and gently to avoid smushing the vegetables. Served with the rest of the cilantro sprinkled over the top and garnished with the optional lemon or lime wedges.