Heat the oil in a medium Dutch oven or heavy pot over medium heat. Add the cumin seeds and cook for about 30 seconds, until fragrant and sizzling.
Add the onion and cook, stirring often, for 4 to 5 minutes, until softened and lightly golden at the edges. Stir in the garlic and ginger and cook for 30 seconds.
Add the tomato, turmeric, ground coriander, and Kashmiri chili powder. Cook for 2 to 3 minutes, stirring and pressing the tomato with the spoon, until it softens into a thick, fragrant base.
Stir in the rinsed red lentils, water, and salt. Bring the pot to a boil over medium-high heat, stirring once or twice so the lentils do not settle on the bottom.
Lower the heat to maintain a gentle simmer. Partially cover the pot and cook for 15 to 18 minutes, stirring occasionally, until the lentils are very tender and the dal is creamy.
Stir in the garam masala and lemon juice. For a looser consistency, stir in a splash of hot water. Taste and add more salt if needed. Serve hot with cilantro, rice, naan, or flatbread.