Heat the olive oil in a large heavy-bottomed pot over medium heat until shimmering. Add the chopped onion and sauté, stirring occasionally, until translucent and fragrant, about 5 minutes.
Add the minced garlic and grated fresh turmeric to the onions. Cook together for about 1 minute, until the mixture releases a warm aroma and the garlic is fragrant.
Stir in the diced carrots and celery. Cook for another 5 minutes, until the vegetables start to soften and become slightly tender.
Pour in the vegetable broth and bring the mixture to a gentle boil. Add the rinsed lentils, then reduce the heat to low and cover the pot.
Let the soup simmer uncovered for about 25-30 minutes, stirring occasionally, until the lentils are tender and starting to break down, giving the soup a slightly thickened, velvety texture.
Season the soup with salt and pepper to taste. Squeeze in fresh lemon juice for brightness and stir well.
If you prefer a creamier texture, use an immersion blender to puree a portion of the soup directly in the pot, or transfer some to a blender and blend until smooth.
Taste and adjust seasoning as needed. Ladle the soup into bowls, garnish with chopped herbs, and a drizzle of olive oil if desired.