Heat the oil in a heavy-bottomed saucepan and saute the onion and half the garlic over low heat until the onion is tender and beginning to color.

Add the lentils, cumin, and coriander, cover by 1 inch with water, and bring to a simmer.

Cover and simmer over low heat for 30 minutes, checking the water from time to time and adding more if the lentils dry out.

Add the remaining garlic, salt and pepper to taste, and cayenne, continue to simmer until the lentils are soft and beginning to disintegrate.

Mash with the back of a spoon stirring often to prevent sticking, or coarsely puree in a food processor.

Correct seasonings and serve, adding the lemon juice if you wish. The puree will keep 3 or 4 days.