Rinse the lentils under cold water until the water runs clear, then set aside.
Heat the olive oil in a large pot over medium heat until it shimmers and begins to ripple.
Add the diced onion to the pot and sauté, stirring frequently, until it becomes translucent and fragrant, about 5 minutes.
Stir in the diced carrots and celery, cooking together until they start to soften and release their aroma, about 7 minutes.
Sprinkle in the ground turmeric and cook for about 30 seconds, stirring constantly until the spice releases its warm aroma.
Pour in the vegetable broth and add the rinsed lentils to the pot. Bring the mixture to a gentle simmer.
Reduce the heat to low, partially cover the pot, and let it simmer gently for 25-30 minutes, until the lentils are tender and the vegetables are soft.
Check the soup’s consistency and seasoning, adding salt and a squeeze of lemon juice if desired, then stir well.
Allow the soup to rest for a few minutes to let the flavors meld, then give it a final stir.
Serve the soup hot, garnished with a drizzle of olive oil or fresh herbs if you like.