Soak the peas in enough water to cover generously for 1 hour.
Drain and place the peas and the turmeric in a large pot with 5 cups water. Bring
to a boil, stirring occasionally. Reduce the heat and simmer partially covered for
45 minutes, stirring occasionally.
Remove from the heat and beat with a whisk or wooden spoon until pureed. Stir
in the salt. (Note: The pea mixture may be prepared in advance up to this point,
and kept refrigerated for up to 4 days, or frozen almost indefinitely.)
Reheat the dal immediately before serving, adding a little water if necessary
to make it the consistency of thin mashed potatoes.
Heat the oil in a skillet over moderate heat until the oil is almost smoking.
Add the cumin seeds and fry for about 10 seconds.
Add the onions and fry, stirring frequently, until they are dark brown,
about 20 minutes.
Add the optional cayenne pepper if you are using it.
Pour the oil and onion mixture
over the split pea puree and serve immediately, garnished with chopped cilantro.