Start by rinsing the red lentils under cold water until the water runs clear, then set aside.
Heat the oil in a wide, shallow pan over medium heat until shimmering and slightly fragrant.
Add the chopped onions and cook, stirring occasionally, until they become translucent and soften, about 5 minutes. You’ll notice a gentle sizzle and the onions turning a light golden hue.
Stir in the minced garlic and ginger, cooking for about 1 minute until fragrant and aromatic, filling the kitchen with a spicy, earthy aroma.
Pour in the diced tomatoes and cook, stirring occasionally, until they break down and the mixture begins to smell sweet and slightly smoky, about 5-7 minutes.
Toast the cumin seeds in the pan for about 30 seconds until they start to pop and release a warm, nutty aroma.
Add the rinsed lentils to the pan, stirring to coat them with the tomato mixture and spices.
Pour in the water or broth, then bring the mixture to a gentle boil, watching as bubbles form and the liquid turns slightly frothy.
Reduce the heat to low and let it simmer uncovered, stirring occasionally, until the lentils soften and the sauce thickens, about 20-25 minutes. The lentils should break down into a velvety, amber-hued sauce.
Stir in ground coriander, turmeric, and season with salt to taste. Cook for another 5 minutes to allow the flavors to meld.
Finish by squeezing fresh lemon juice into the curry and stirring in chopped cilantro for a burst of brightness and freshness.
Serve the warm, velvety curry over rice or with flatbread, garnished with extra cilantro if desired. Enjoy its comforting aroma and rich, hearty flavor.