Pick over, clean, and wash lentils for 30 seconds in a sieve under running water or until the water runs clear.
Put the lentils into a deep pot along with the turmeric and water, and bring to a boil, stirring often, as the lentils have a tendency to lump together at this stage. Reduce heat to medium-low and simmer, partially covered, for 25--30 minutes (or 40--45 minutes for yellow lentils) or until the lentils are thoroughly cooked and tender when pressed between your fingers. Stir now and then to prevent sticking.
Turn off the heat and beat the lentils with a wire whisk or wooden spoon for a minute to smooth the puree. Measure. There should be 5--5.5 cups of lentil puree. If you have less, add sufficient water to make 5 cups. Stir in the salt. (This puree may be prepared ahead and refrigerated for up to three days. It also freezes well. Defrost thoroughly before proceeding with the recipe.)
When ready to serve, simmer the puree in the pot over low heat until piping hot. The puree thickens with keeping; therefore check the consistency and add water if necessary. It should be like a moderately thick cream soup. Keep the puree warm while you make the garlic butter (tadka).
Heat the oil over medium heat in a small frying pan. When it is hot, add the garlic slices, and fry just until they turn light brown and are still soft (about 1--2 minutes). Turn off heat and immediately pour this perfumed garlic butter with the slivers over the lentil puree. Stir to mix, and serve with bread or rice.