In a large saucepan or dutch oven heat oil and add lamb over medium heat. Cover and brown meat, stirring occassionally. When brown remove from heat and let cool for several minutes.
Slowly add 4 cups water and bay leaves. Return to heat and let simmer over low heat.
Meanwhile in a cast iron skillet roast rinsed chiles for a few minutes. Remove and place in a stainless stell bowl or small pan.
Add one cup of broth from the simmering lamb. Let soak until soft (10-15 minutes).
Poke tomatoes once near the stems and place in cast iron skillet (use the same one used for chiles) and char them lightly by rolling them on their sides. Remove from heat and let cool before cutting out the stems.
Using a blender, puree the chiles with broth, tomatoes, onion, garlic and spices. When pureed add mixture and potatoes to simmering lamb.
Continuing cooking until potatoes are soft about 30-45 minutes.