Heat the Oil: In a large pot or Dutch oven, heat the olive oil over medium heat.
Sauté the Aromatics: Add the diced onion and cook for about 5 minutes, or until softened. Stir in the minced garlic and cook for another 1-2 minutes until fragrant.
Add Vegetables: Add the carrots, celery, and bell pepper to the pot. Cook for about 5 minutes, stirring occasionally, until the vegetables start to soften.
Combine Lentils and Potatoes: Stir in the lentils and diced potato, cooking for an additional 2 minutes.
Add Liquids and Seasonings: Pour in the diced tomatoes and vegetable broth. Stir in the ground cumin, dried thyme, smoked paprika, and bay leaf. Season with salt and black pepper to taste.
Simmer: Bring the soup to a boil, then reduce the heat to low. Cover and simmer for about 30 minutes, or until the lentils and vegetables are tender.
Add Greens: If using, stir in the fresh spinach or kale and cook for an additional 2-3 minutes, until wilted.
Finish with Lemon Juice: If desired, stir in the lemon juice for a burst of freshness.
Adjust Seasoning: Taste the soup and adjust seasoning with more salt, pepper, or additional spices if needed.
Rest and Serve: Allow the soup to rest for about 10 minutes before serving to let the flavors meld.