Squeeze as much water as possible from spinach and artichoke hearts by pressing in a colander. The more liquid you squeeze out, the thicker and more flavorful the dip will be.
Chop spinach and artichokes coarsely then place in a large bowl. Stir in softened cream cheese until smooth and well combined. Add sour cream, mayonnaise, milk, pepper jack, Parmesan, garlic powder, red pepper flakes, salt and pepper to taste then mix thoroughly.
Lightly coat inside of slow cooker with cooking spray then spread dip evenly into bottom of the dish. Cook on high for 2 hours, or until sides are brown and center is hot. If you would like to cook this on low heat, leave it in for 4 hours.
Stir to recombine ingredients then taste for salt and pepper. Add a bit more milk if necessary to thin the dip. Keep dip on a warm setting in the slow cooker or transfer to a bowl and serve immediately.
Not only does the slow cooker bake the dip, but it also keeps it warm during the party. We call that a win-win!