Heat the oil in a heavy saucepan until a haze forms on top. Add the cinnamon, cardamom, garlic, and red chiles. As soon as the spices swell and change color, add the onions and saute until well browned. Add the lamb and cook, turning occasionally, until browned on all sides. When the liquid in the pan has almost dried up, add the ground spices and the ginger/garlic paste and stir well. Then add some salt and enough water (or stock) to just about cover the lamb. Cover with a tight-fitting lid and summer for 15 to 20 minutes, stirring from time to time and making sure the sides of the pan are kept clean.
Add the potatoes and mix them in well, then add the tomato, if using. Cover again and simmer until the lamb and potatoes are tender. Check the seasoning and sprinkle with some chopped fresh cilantro. Stir some in as well, if you like, for an extra kick.
Ginger/Garlic Paste: Ginger/garlic paste forms the basis of most of India's cooking. In many recipes in this book I have listed finely chopped garlic and ginger separately to make life easier. However, in some recipes, only a paste will do. Most supermarkets now sell separate jars of ginger paste and garlic paste. All you need to do is blend them together.
To make your own, take equal quantities of peeled garlic and fresh ginger and work them to a paste with a mortar and pestle. Alternatively, puree them in a blender with a little water and some bland vegetable oil (not olive oil), stopping regularly to scrape down the sides. The paste will keep in the refrigerator for up to 2 months if you mix in a little oil to prevent spoiling.