In a large pot, heat the olive oil over medium heat. Add the diced onion and sauté until translucent, about 5 minutes.
Stir in the minced garlic, carrots, celery, and red bell pepper. Cook for an additional 5-7 minutes, until the veggies are slightly softened.
Add the rinsed lentils to the pot along with the diced tomatoes (with their juice), vegetable broth, ground cumin, smoked paprika, and dried oregano. Stir well to combine.
Bring the mixture to a boil. Once boiling, reduce the heat to low, cover, and let simmer for about 30-35 minutes, or until the lentils are tender.
After simmering, use an immersion blender to blend the soup slightly for a creamier texture, if desired. Alternatively, you can blend a portion of the soup in a regular blender and return it to the pot.
Season the soup with salt, pepper, and lemon juice. Stir well and let it heat through for another 5 minutes.