In a large pot, heat the olive oil over medium heat. Add the diced onion and garlic, sautéing until the onion is translucent, about 3-4 minutes.
Add the diced carrots, celery, and bell pepper to the pot. Cook for an additional 5 minutes, stirring occasionally.
Stir in the zucchini, lentils, diced tomatoes (with juice), vegetable broth, thyme, oregano, bay leaf, salt, and pepper. Bring the mixture to a boil.
Once boiling, reduce the heat to low and cover the pot. Let the soup 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, add lemon juice for brightness.
Taste and adjust seasoning if necessary. Serve hot.