Heat the heavy-bottomed pot over medium heat and add the oil. Once shimmering, add the diced onion and minced garlic, sautéing until the onion turns golden and fragrant, about 5 minutes.
Add the chopped carrots and cook for another 3-4 minutes, stirring occasionally until they begin to soften and smell sweet.
Sprinkle in the ground turmeric and toast it with the vegetables for about 30 seconds, releasing its warm aroma.
Pour in the vegetable broth and bring the mixture to a gentle boil. Add the rinsed lentils, stir, then reduce the heat to low, cover, and let simmer for 25-30 minutes until the lentils are tender and the soup thickens.
Use an immersion blender to puree part of the soup directly in the pot, or transfer half to a blender and blend until smooth. Return to the pot if blended separately. This creates a creamy, comforting texture while leaving some vegetables intact.
Season with salt to taste, then squeeze in the fresh lemon juice and stir well. Taste and adjust seasoning if needed.
Let the soup rest off the heat for a few minutes to allow flavors to meld, then ladle into bowls and serve hot, garnished with herbs if desired.