Rinse the lentils thoroughly in cold water until the water runs clear, then set aside.
Heat a tablespoon of olive oil in a large pot over medium heat until it shimmers and begins to smell fragrant.
Add the chopped onion to the pot and sauté for about 5 minutes, stirring occasionally, until it turns translucent and starts to soften.
Stir in the minced garlic and ground cumin (or smoked paprika) and cook for another minute until the spices release their aroma and the garlic turns golden.
Add the diced carrots and celery, stirring well, and cook for 3-4 minutes until they begin to soften and the mixture smells sweet and earthy.
Pour in the vegetable broth and add the rinsed lentils. Bring the mixture to a gentle boil, listening for bubbling sounds.
Reduce the heat to low, partially cover the pot, and let it simmer gently for 25-30 minutes, or until the lentils are tender and have absorbed the flavors.
Check the consistency; if the soup is too thick, add a splash more broth or water and stir gently.
Season with salt to taste, then stir in the lemon zest and squeeze in the lemon juice for brightness.
Use an immersion blender to puree about half of the soup directly in the pot, creating a creamy and hearty texture while leaving some lentils whole for texture.
Taste once more and adjust seasonings as needed. Let the soup rest for 10 minutes to allow flavors to meld.
Serve the lentil soup in bowls, garnished with chopped fresh herbs and an extra squeeze of lemon if desired. Enjoy the comforting, velvety texture and earthy aroma.