Combine 1/2 cup spinach liquid, milk, and garlic in a 1-quart microwave-safe measuring cup or small bowl covered with a saucer; microwave (or heat slowly in a medium saucepan) until very hot and steamy. Let stand until garlic softened, 5 to 10 minutes.

Melt butter in a Dutch oven over medium-high heat; whisk in flour. Add hot milk mixture all at once; 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 Parmesan cheese, nutmeg, salt, and black pepper. Serve hot.