Make a paste of the lime leaves, chilies, garlic, ginger, cilantro stems, cumin seeds, salt, and fish sauce. (add everything but fish sauce to mortar and pestle, and grind. Then add fish sauce)
sweat onions until translucent (5 - 10 minutes medium heat)
Add paste to onions and cook for 1 minute to remove raw edge
Add lentils, broth, coconut milk, bring to a boil, and redice to simmer until lentils are tender (30 minutes? with the lid on?)
stir in the cilantro
Notes
if using a food processor to make the paste grind the chillis in a spice grinder seperately