Heat a tablespoon of oil in a heavy-bottomed pot over medium heat until shimmering, and then add finely chopped onions. Cook for about 5 minutes, stirring often, until they become translucent and start to turn golden at the edges.
Add minced garlic and grated ginger to the onions. Cook for 1 minute, stirring constantly, until fragrant and aromatic, filling the kitchen with a spicy, sweet smell.
Sprinkle in toasted cumin seeds, turmeric, ground coriander, and chili powder. Stir for about 30 seconds until the spices release their aroma and darken slightly, coating the onion mixture evenly.
Add the diced tomatoes to the pot. Stir well and cook for 5 minutes, pressing the tomatoes with the back of your spoon to help them break down. The mixture should thicken and darken in color.
Pour in the rinsed red lentils and stir to coat them in the spice mixture. Then, add the vegetable broth and bring everything to a gentle boil.
Reduce the heat to low, cover the pot, and let the curry simmer gently for about 20 minutes. During this time, the lentils will soften and break apart, creating a thick, velvety sauce.
Check the lentils—they should be tender and mash easily. Stir in the juice of a lemon to brighten the flavors, and taste to adjust salt or spice if needed.
Cook for another 5 minutes uncovered if you want a thicker sauce. Then, remove from heat and garnish with chopped fresh cilantro.
Serve the warm curry in bowls, paired with rice or flatbread, and enjoy the comforting aroma and silky texture.