In a slow cooker, place all ingredients except mushrooms, spinach, and tomato paste. Stir well to combine.
Cover and cook on LOW for 6 to 8 hours or until beef strips are fork-tender and veggies are done. Stir in mushrooms and cook for 15 minutes or until just tender.
Stir in spinach and cook for 3 to 5 minutes or until just wilted. Stir in tomato paste to thicken slightly, if desired. Blend well then serve immediately.