Wash lentils in a strainer. Combine lentils and water in a saucepan over medium-high heat. Add 1/2 of the sliced onions and garlic, reserving the rest for later. Stir in turmeric, bay leaf, tomatoes, and salt. Add chiles, leaving them whole to add flavor or cut in half to add heat. When mixture begins to boil, reduce heat to a simmer; cook until lentils break apart and thicken slightly, about 20 minutes.

Meanwhile, in a skillet, heat vegetable oil over medium heat until oil shimmers. Add reserved sliced onions; cook and stir until onion has softened and translucent, about 5 minutes. Reduce heat to medium-low; continue cooking and stirring until onion is very tender and dark brown, 15 to 20 minutes more. Stir in reserved garlic and cook, stirring constantly, until garlic is fragrant and tender, about 2 minutes.