Heat oil in a heavy-bottomed pot over medium heat until it shimmers. Add cumin seeds and cook for about 30 seconds until fragrant and crackling.
Add the chopped onion and cook, stirring occasionally, until translucent and soft, about 5 minutes. The onion should be fragrant and slightly golden around the edges.
Stir in the minced garlic and cook for another minute until fragrant, filling your kitchen with a warm, inviting aroma.
Add ground turmeric and cumin powder, stirring well to toast the spices for about 1 minute. You’ll notice a deep, earthy fragrance as they release their oils.
Pour in the vegetable broth and bring the mixture to a gentle simmer, allowing the flavors to meld for about 10 minutes.
Add the rinsed red lentils, diced carrots, and chopped celery to the pot. Stir to combine all ingredients evenly.
Reduce the heat to low and let the soup simmer uncovered for 25-30 minutes, stirring occasionally, until the lentils have broken down and the soup is thick and velvety.
Taste the soup and season with salt and pepper as needed. Stir in the lemon juice for a bright, fresh kick of flavor.
Just before serving, sprinkle chopped cilantro or parsley over the top for a burst of freshness and color.
Serve the soup hot in bowls, enjoying the velvety texture and vibrant aroma that transports you to a cozy kitchen in India.