Rinse the red lentils under cold water until the water runs clear, then set them aside.
Heat the vegetable oil in a large pot over medium heat. Add the mustard seeds and cumin seeds. Cook until the mustard seeds begin to pop, about 30 seconds.
Stir in the chopped onion and cook until it softens, about 4 minutes. Add the minced garlic and cook for 1 minute more.
Sprinkle in the turmeric and salt, then stir to coat the onion mixture.
Add the rinsed lentils and water to the pot. Bring the mixture to a boil.
Reduce the heat to low, cover the pot, and simmer for 20 minutes, stirring once or twice, until the lentils are tender and the soup has thickened.