Dice the onion, celery, and carrot into small, even pieces so they cook at the same rate. Peel and finely mince the garlic and ginger, then mince the jalapeño. (For a milder flavor, remove the seeds and white membrane from the jalapeño before chopping.)
Heat the olive oil in a large soup pot over medium heat. Add the onion, carrot, and celery, stirring often.
Cook for 5–7 minutes until the onion turns translucent and the vegetables begin to soften. Take your time here, as this builds the soup’s flavor base.
Stir in the garlic, ginger, and jalapeño, cooking for about 3 minutes until fragrant. Add the cumin, turmeric, tomato paste, and half the salt.
Stir until everything is coated and the mixture looks like a thick paste clinging to the vegetables.
Rinse the lentils well, then stir them into the pot so they’re coated with the spices. Slowly pour in the vegetable broth while scraping the bottom of the pot to lift up any browned bits. These add a lot of flavor to the soup.
Raise the heat to bring the soup to a gentle boil, then reduce to a simmer. Cook uncovered for 20–25 minutes, stirring occasionally. The lentils should be very soft and begin to break down, naturally thickening the soup.
Remove the pot from the heat. Stir in the coconut milk and lemon juice until the soup looks creamy and smooth. Taste and adjust with more salt or lemon if needed.
Ladle the soup into bowls and top with fresh cilantro. Pair with warm flatbread or a scoop of steamed rice for a full meal.