Combine the lentils, salt, turmeric with 4 1/2 cups water in a saucepan. Bring to a boil, stir well then lower the heat to simmer partially covered for 15 to 20 minutes.
Heat a small saute pan over medium-high heat with the oil. When the oil is hot, add the onion and cook just until translucent.
Add the whole cumin, mustard, fenugreek, and fennel seeds with the red pepper flakes and stir until the spices become fragrant.
Add the ginger and garlic and stir until fragrant, before stirring in the tomato paste. Cook for 1 to 2 minutes while stirring continuously.
Transfer the onion-spice-tomato mixture into the pot of lentils. Cook for 5 minutes more, uncovered, until the flavors are blended and the lentils are very tender. Or continue cooking until the lentils are the thickness you prefer, 10 to 15 minutes more.