Place a large heavy-bottomed pot over medium heat and add the oil. Once shimmering, add the finely chopped onion and cook, stirring occasionally, until translucent and fragrant, about 5 minutes.
Add the minced garlic and grated ginger to the pot. Cook for about 1 minute, until fragrant and slightly softened, filling the kitchen with a warm aroma.
Stir in the turmeric powder, cumin seeds, and smoked paprika. Toast the spices, stirring constantly, until they shimmer and release their aroma, about 30 seconds.
Add the rinsed red lentils to the pot, stirring to coat them with the spices and oil. Cook for 1-2 minutes to lightly toast the lentils and enhance flavor.
Pour in the vegetable broth and stir well, scraping up any bits stuck to the bottom. Bring the mixture to a gentle boil, then reduce the heat to low and partially cover the pot.
Simmer the curry gently for 20-25 minutes, stirring occasionally, until the lentils are tender and have started to break down, creating a thick, velvety sauce.
Stir in the coconut milk, if using, for added richness. Continue simmering for another 5 minutes to meld all flavors together.
Turn off the heat and squeeze fresh lemon juice into the curry. Taste and adjust seasoning with salt and more lemon if desired.
Garnish with chopped fresh cilantro and serve hot, paired with rice or bread for a complete meal.