Rinse the red lentils thoroughly under cold water until the water runs clear, then set aside.
Heat the oil in a large heavy-bottomed pot over medium heat until shimmering and warm.
Add the chopped onion to the pot and sauté, stirring frequently, until translucent and fragrant, about 5 minutes. You’ll notice a gentle sizzle and a sweet aroma filling the air.
Add the minced garlic and cook for another minute, stirring until fragrant. The mixture will become more aromatic and slightly golden.
Stir in the cumin seeds and turmeric powder, cooking for about 30 seconds until the seeds crackle and spices release their earthy aroma.
Pour in the diced tomatoes and cook, stirring occasionally, until the mixture thickens slightly and the oil begins to separate at the edges, about 10 minutes. You'll see the color deepen and hear a gentle bubbling.
Add the rinsed lentils to the pot, then pour in the water or broth. Stir gently to combine all ingredients evenly.
Bring the mixture to a boil over high heat, then reduce the heat to low, cover the pot, and let simmer gently for about 20 minutes, or until the lentils are soft and breaking apart. You’ll hear a steady simmer and see the curry thickening into a velvety stew.
Uncover and check the consistency; it should be thick and creamy, with the lentils fully cooked and melting into the sauce. Taste and adjust salt as needed.
Squeeze fresh lemon juice into the curry for brightness, then stir in chopped cilantro for a fresh, herbal contrast. The vibrant green and golden hues will make the dish look irresistibly inviting.
Let the curry rest off the heat for a few minutes to allow flavors to meld, then serve hot with rice or bread, garnished with extra cilantro and lemon zest for a lively finish.