Start by washing your brown lentils thoroughly. Pick through them to remove any small stones or debris. Dice the onion finely and chop the celery into small pieces to ensure they soften nicely during cooking. Rinse and chop the spinach, removing the tough stems.
Add the washed lentils, diced onion, chopped celery, minced garlic, petite diced tomatoes (with their juices), and all the dried herbs—Greek seasoning, thyme, and oregano—into your slow cooker. Pour in the vegetable broth and sprinkle with fresh-ground black pepper to taste.
Cover your slow cooker and set it to low. Let the soup cook for 6 to 8 hours, or until the lentils are tender and the flavors have melded beautifully. Alternatively, you can cook on high for about 3 to 4 hours if you’re short on time.
About 15 minutes before serving, stir in the chopped spinach so it wilts gently into the soup. Then add the fresh-squeezed lemon juice to brighten the flavors and give the soup a subtle, tangy finish.
Ladle the soup into bowls and top with crumbled feta cheese if you like. The salty creaminess of the feta perfectly complements the tangy, herby broth. Serve with warm bread or pita for a complete meal.