Combine milk, spinach liquid and garlic in a 1-quart microwave-safe measuring cup or small bowl covered with a saucer, and microwave (or heat slowly in a medium saucepan) until very hot and steamy.
Let stand for 5 to 10 minutes to soften garlic.
Melt butter in a Dutch oven over medium-high heat.
Whisk in flour.
Add hot milk mixture all at once, then whisk until smooth. Stir in spinach; cook until sauce is thick and bubbly and spinach is tender but still green, about 5 minutes. Stir in cheese, nutmeg, and a generous sprinkling of salt and pepper.
