Rinse and wipe out the saucepan, then add the arrowroot, milk, and water or broth and whisk until well combined. Bring to a simmer over medium-low heat, stirring constantly, and cook for a couple of minutes, until the mixture begins to thicken. Add the cheese, turn down the heat to low, and cook, stirring, until the cheese has melted. Stir in the spinach and Anaheims. Taste and adjust the seasonings, if you like.