Dice the onion, carrots, celery stalks, and slice the Baby Bella mushrooms. Finely dice the garlic.
Add all that to the slow cooker. Add the lentils, vegetable broth, diced tomatoes, salt, pepper, and cumin to the slow cooker and stir.
Set the crock pot to low and allow to cook until the vegetables are tender, but not falling apart, about 7 1/2 hours.
Remove two cups of soup and add to a blender, and hit pulse a few time to slightly puree it. Then add it back to the slow cooker. You can also use an immersion blender to the same effect.
Add the spinach and set the slow cooker to high for 30 more minutes.
Serve, season with salt and pepper to taste.