Prep the veggies. Peel and roughly chop the onion. Remove the ends from the zucchini and yellow squash, cut them in half lengthwise, and then slice 1/4 to 1/2 inch thick.
Add the onion, zucchini, yellow squash, diced tomatoes, garlic, and vegetable broth to the slow cooker.
Cover and cook on HIGH for 4 hours or LOW for 8 hours.
A few minutes before serving, stir in the baby spinach and allow it to wilt.
Season with salt and pepper to taste, ladle the soup into bowls, and enjoy!