Warm a tablespoon of oil in a large heavy-bottomed pot over medium heat until shimmering and fragrant.
Add chopped onions, carrots, and celery to the pot. Cook for about 5-7 minutes, stirring occasionally, until the vegetables are soft and translucent, filling the kitchen with a sweet aroma.
Stir in the ground turmeric and cook for another minute, allowing the spice to bloom and release its warm fragrance.
Add rinsed red lentils to the pot, stirring to coat them with the spices and vegetables.
Pour in the vegetable broth, then bring the mixture to a gentle boil over high heat, watching for bubbles to rise and the aroma to intensify.
Reduce the heat to low and let the soup simmer uncovered for about 20-25 minutes, stirring occasionally, until the lentils are tender and the broth slightly thickened.
If you prefer a smoother texture, use an immersion blender directly in the pot to blend part or all of the soup until velvety.
Add chopped greens in the last 5 minutes of cooking, stirring until they wilt and brighten the soup with fresh color and flavor.
Taste the soup and season with salt as needed, adjusting the flavors to your liking.
Once the lentils are tender and the flavors meld, turn off the heat and let the soup rest for a few minutes to deepen the taste.
Scoop the fragrant, vibrant soup into bowls, garnish with a squeeze of lemon or fresh herbs if desired, and enjoy the comforting warmth with every spoonful.