Heat olive oil in a large pot or Dutch oven over medium heat. Add the chopped onion, carrots, and celery. Cook, stirring occasionally, for 6-8 minutes, until softened.

Add the minced garlic, dried thyme, and dried oregano. Cook for 1 minute more, until fragrant. Stir in the tomato paste and cook for another minute.

Add the rinsed lentils, diced tomatoes (with their juices), and vegetable broth. Season with salt and pepper. Bring to a boil, then reduce the heat to low, cover, and simmer for 25-30 minutes, or until the lentils are tender.

Stir in the fresh spinach and cook for 2-3 minutes until it has wilted. Remove from heat and stir in the fresh lemon juice. Taste and adjust seasoning with more salt, pepper, or lemon juice if needed. Serve hot.