Add the lentils, onion, carrots, celery, zucchini, vegetable broth, diced tomatoes, garlic, oregano, and basil to the slow cooker.
Stir everything together and season with salt and pepper.
Cover and cook on low for 6 to 8 hours until the lentils are soft.
Stir in the spinach during the final 10 minutes so it wilts but stays bright.
Taste the soup and adjust salt or pepper if needed before serving.
Notes
Add the spinach only at the end to keep its color and texture. The soup thickens as it sits, so stir in a little extra broth when reheating leftovers. It keeps in the fridge for four days and freezes well in single portions.Course: Soup
Cuisine: Italian
Equipment: Slow Cooker