The water from the spinach needs to be removed. Do this by adding the thawed spinach to a clean tea towel or dish towel. Gather the towel around the spinach and wring out the liquid into the sink or a bowl.
Add the strained spinach to the slow cooker. Add the remaining ingredients. Stir. You won't be able to mix this all the way, that's ok. Just a rough stir is fine.
Place the lid on the slow cooker. Cook on HIGH for 2.5-3 hours or LOW for 4 hours. Stirring occasionally. The dip is done when it is piping hot.