Turn the slow cooker on high. Put the olive oil, onions, peppers, and carrots inside and let them cook for a couple of minutes while you check the lentils for debris and rinse them.
Add the rinsed lentils and the brown rice. Add the spices and most of the box of broth. Reserve about ¾ of a cup in case you need to add a little bit more liquid near the end of the cooking time.
Cover and cook on high for three hours.
Serve in a bowl with fresh spinach leaves on top and more salt and pepper as desired.