In a large skillet, brown ground beef and onions until meat is browned and crumbly; drain. Add taco seasoning and ½ cup water, and cook until all water is absorbed.
Spray slow cooker with cooking spray. Add ¾ cup salsa to the bottom of the slow cooker; cover with a layer of tortillas.
Place half of meat mixture over tortillas; top with dollops of cream cheese (using half of the carton), 1 cup spinach, ½ cup Mexican blend cheese and ¾ cup salsa. Repeat layers.
Finish with a layer of tortillas, cover with remaining salsa and cheese.
Cover, and cook on low 5-6 hours or on high 3-4 hours.
Turn slow cooker off 10-15 minutes before serving.
Serve with desired toppings.