In a large pot, heat the olive oil over medium heat. Add the diced onion and sauté for about 3-4 minutes until translucent.
Stir in the minced garlic, carrots, celery, and bell pepper. Cook for another 5 minutes, stirring occasionally.
Add the zucchini, diced tomatoes (with juice), lentils, vegetable broth, thyme, oregano, bay leaf, salt, and pepper. Bring the mixture to a boil.
Reduce the heat to low, cover, and let it simmer for about 30-35 minutes, or until the lentils are tender.
Remove the bay leaf and stir in the chopped spinach or kale. Cook for an additional 5 minutes until the greens are wilted.
If desired, stir in the lemon juice for added brightness before serving.