Heat the olive oil in a large pot or Dutch oven over medium heat. Add the diced onion, carrots, and celery. Cook for about 6 to 8 minutes, stirring occasionally, until the vegetables begin to soften and the onions become translucent.
Stir in the minced garlic, dried oregano, ground cumin, and smoked paprika. Cook for just 1 minute until the spices are fragrant. Be careful not to burn the garlic, as it can turn bitter.
Add the rinsed lentils, diced tomatoes (with their juices), and the vegetable broth to the pot. Increase the heat to bring the liquid to a boil, then reduce the heat to low. Cover the pot and let it simmer for 25 to 30 minutes, or until the lentils are tender but not mushy.
Once the lentils are cooked, stir in the baby spinach and let it wilt for about 1 minute. Turn off the heat and stir in the lemon juice. Season with salt and black pepper to taste. The lemon juice is key as it brightens all the earthy flavors.