Combine Base Ingredients: In a medium saucepan or small soup pot, add the rinsed lentils, chopped onion, diced carrot, diced celery, minced garlic, low-sodium vegetable broth, water, no-salt-added diced tomatoes, olive oil, Italian seasoning, red pepper flakes (if using), and the bay leaf. Stir gently to combine.
Bring to a Boil: Place the pot over high heat and bring the mixture to a rolling boil.
Simmer the Soup: Once boiling, immediately reduce the heat to medium-low. Cover the pot and let the soup simmer gently for 25 to 30 minutes, stirring occasionally, until the lentils are tender and fully cooked.
Add Spinach: After the lentils are tender, turn off the heat. Add the fresh spinach to the hot soup. Stir for about a minute, or until the spinach has wilted completely.
Finish and Serve: Remove and discard the bay leaf. Stir in the fresh lemon juice (this step is crucial to brighten the flavors). Taste and season generously with freshly ground black pepper. Ladle the hot soup into two bowls and serve immediately.