Add the beef stew meat, lentils, diced tomatoes with their juices, beef broth, onion, carrots, celery, garlic, thyme, salt, and pepper to the slow cooker.
Stir everything together until the ingredients are evenly distributed.
Cover and cook on low for 7 to 8 hours, until the beef is tender and the lentils are soft.
Stir in the fresh spinach during the last 10 minutes of cooking so it wilts but keeps its color.
Taste and adjust salt or pepper if needed before serving.
Notes
Stir the spinach in only at the end so it stays bright green instead of turning dull. If the soup thickens too much overnight, add a splash of broth when reheating. Leftovers keep well in the fridge for three days and taste even better the next day as the flavors settle.Course: Soup
Cuisine: American
Equipment: Slow Cooker, Knife, Cutting Board