Start by rinsing the red lentils under cold water until the water runs clear, then set them aside.
Heat the oil in a large saucepan over medium heat until it shimmers and starts to smell toasty.
Add the chopped onion and cook, stirring frequently, until it becomes translucent and soft, about 5 minutes.
Stir in the minced garlic and grated ginger, cooking for about 30 seconds until fragrant.
Add the ground cumin, turmeric, and chili powder, stirring constantly to toast the spices and release their aroma, about 1 minute.
Pour in the vegetable broth or water, bringing it to a gentle simmer.
Stir in the rinsed lentils, ensuring they are submerged in the liquid.
Reduce the heat to low, cover the pan, and let the lentils simmer gently for 20-25 minutes, stirring occasionally until tender and the sauce thickens.
Uncover the pan, stir in the coconut milk, and cook uncovered for another 5 minutes to let the flavors meld and the sauce thicken further.
Squeeze fresh lemon juice into the curry, tasting and adjusting salt and spice as needed. The curry should be fragrant, thick, and creamy, with a vibrant orange hue.
Remove from heat and let it rest for a few minutes. Serve hot over rice or with flatbread, garnished with herbs if desired.