Heat olive oil in a large pot over medium heat. Sauté onion and carrots until onion becomes translucent, about 5-7 minutes.
Add minced garlic and cook for an additional 2-3 minutes until fragrant.
Stir in chopped spinach and cook until wilted, about 2-3 minutes.
Add rinsed lentils, oregano, diced tomatoes, and vegetable broth to the pot.
Bring mixture to a boil, then reduce heat to low and simmer for 20-30 minutes until lentils are tender.
Remove from heat and stir in fresh lemon juice.
Season with salt and pepper to taste and serve warm.