Heat the olive oil in a large pot or Dutch oven over medium heat.
Add the diced onion and sauté for about 5 minutes until translucent.
Stir in the minced garlic and cook for an additional minute until fragrant.
Add the diced carrots, celery, and bell pepper. Sauté for about 5-7 minutes until the vegetables start to soften.
Stir in the rinsed lentils, vegetable broth, diced tomatoes (with juice), cumin, thyme, and smoked paprika. Season with salt and pepper.
Bring the mixture to a boil, then reduce the heat to low. Cover and let it simmer for about 25-30 minutes or until the lentils are tender.
Just before serving, stir in the chopped spinach or kale and cook for another 2-3 minutes until wilted.
If desired, add the lemon juice for a bright finish. Taste and adjust the seasoning if necessary.
Serve hot, ladled into bowls. Enjoy!