Bring a large pot of salted water to a boil. Add the spinach and cook just until wilted, about 1 minute. Drain in a colander; rinse with cold water until cool. Squeeze the spinach to remove as much liquid as possible; coarsely chop, and set aside.
In a large saucepan, heat the butter over medium heat. Add the onion and garlic; season with salt and pepper. Cook, stirring occasionally, until the onion softens, 3 to 5 minutes.
Add the cream cheese and milk; cook, stirring, until the cream cheese is melted and smooth. Stir in the spinach. Simmer over medium heat until the mixture thickens, 8 to 10 minutes. Add the nutmeg; season with salt and pepper.
Notes
Using reduced-fat cream cheese and low fat milk is perfectly okay.
For bulk amounts of spinach, it will be cheapest at a store like Costco.