Heat the oil in a heavy-bottomed pot over medium heat until shimmering and fragrant.
Add the chopped onion and cook, stirring often, until translucent and slightly golden, about 5 minutes. You should hear a gentle sizzle and see the onions soften.
Stir in the minced garlic and grated ginger, cooking for about 1 minute until fragrant and slightly darker but not browned.
Add the cumin seeds and toast them in the oil for about 30 seconds until aromatic and slightly darker, releasing a warm, nutty smell.
Stir in the ground turmeric and coriander, cooking for another 30 seconds to wake up the spices' aroma.
Add the rinsed red lentils and diced tomatoes to the pot, stirring well to coat everything evenly with spices and aromatics. Cook for a minute until the tomatoes start breaking down and releasing their juices.
Pour in the water or broth, bringing the mixture to a gentle boil, then reduce the heat to low, cover partially, and let simmer for 20-25 minutes, stirring occasionally. The lentils should become soft and start breaking apart, thickening the curry.
Check the seasoning and add salt to taste. Squeeze in the lemon juice to brighten the flavors, stirring well.
Garnish the curry with chopped cilantro just before serving for a fresh herbal note.
Serve hot over steamed rice or with warm flatbread, enjoying the comforting texture and rich aroma.