Heat a large pot over medium heat, and add a splash of oil. Once shimmering, add diced onions and cook, stirring often, until translucent and fragrant, about 5 minutes.
Add minced garlic and turmeric to the pot. Stir constantly for about 30 seconds until the mixture is fragrant and the turmeric begins to turn a deeper yellow.
Stir in chopped carrots and cook for another 5 minutes, allowing them to soften slightly and release their sweet aroma.
Pour in the vegetable broth and add diced tomatoes, stirring to combine. Bring the mixture to a gentle boil.
Add the rinsed red lentils, season with salt and pepper, then partially cover the pot. Reduce heat to low and simmer for 25 to 30 minutes, stirring occasionally, until the lentils are soft and the soup has thickened slightly.
If you prefer a smoother texture, use an immersion blender to blend part of the soup directly in the pot until creamy. Otherwise, leave it chunky for more texture.
Give the soup a final taste and adjust salt, pepper, or spice as needed. Let it sit for a few minutes to allow flavors to meld.
Serve the soup hot, garnished with fresh herbs if desired, alongside crusty bread or a simple salad for a complete meal.