Brown the sausage slices in a skillet over medium heat until lightly golden on both sides. Drain any excess fat and transfer the sausage to the slow cooker.
Add the lentils, onion, carrots, garlic, chicken broth, diced tomatoes, oregano, salt, and pepper to the slow cooker. Stir to combine all ingredients.
Cover and cook on low for 7 to 8 hours or on high for 4 hours until the lentils are tender.
Stir in the fresh spinach during the last 10 minutes of cooking so it wilts but stays bright.
Notes
If the soup thickens too much after cooking, add a splash of broth or water to loosen it before serving. Leftovers reheat well on the stovetop with a little extra liquid. For a milder flavor, swap mild sausage for the Italian variety.Course: Soup
Cuisine: Italian
Equipment: Slow Cooker, Large Skillet