Start by chopping a large onion into small, uniform pieces. Heat a large pot over medium heat and add two tablespoons of olive oil. Once shimmering, add the chopped onion and sauté until translucent and fragrant, about 5 minutes.
Add diced carrots and celery to the pot, along with a teaspoon of turmeric powder. Stir well and cook for another 5-7 minutes until the vegetables soften slightly and begin to release their sweet aroma.
Stir in rinsed lentils, ensuring they are coated with the spices and vegetables. Pour in 1.5 liters of vegetable broth and increase the heat to bring the mixture to a gentle boil.
Once boiling, reduce the heat to low, cover the pot, and let it simmer for about 30 minutes. Stir occasionally, and you'll notice the lentils breaking down and the soup thickening with a comforting, velvety texture.
Uncover the pot and taste the soup. Adjust the salt and add a squeeze of fresh lemon juice to brighten the flavors. If the soup is too thick, stir in a splash of hot water or broth until it reaches your desired consistency.
Give the soup a final stir, then let it rest off the heat for a few minutes. Ladle into bowls, garnish with fresh herbs if desired, and serve hot for a nourishing, comforting meal.