Heat the olive oil in a large pot or Dutch oven over medium heat.
Add the diced onion, carrots, and celery to the pot. Sauté for 8 to 10 minutes until the vegetables are softened and the onion is translucent.
Stir in the minced garlic and ground cumin, cooking for 1 additional minute until fragrant.
Pour in the vegetable broth. Add the rinsed lentils, black pepper, and salt. Stir well to combine and bring the mixture to a gentle boil.
Reduce the heat to low, cover the pot with a lid, and simmer for 35 to 40 minutes, or until the lentils are completely tender.
For a creamier texture, use an immersion blender to pulse a small portion of the soup, leaving the majority of it chunky.
Remove the pot from the heat. Stir in the chopped fresh parsley and lemon juice.
Ladle the hot soup into bowls and serve immediately.