Step #1
Combine milk, spinach liquid & garlic in a 1-quart microwave-safe measuring c or small bowl covered with a saucer, & microwave (or heat slowly in a medium saucepan) until very hot & steamy.
Step #2 Let stand for 5 to 10 mins to soften garlic.
Step #3 Melt butter in a Dutch oven over medium-high heat.
Step #4 Whisk in flour.
Step #5 Add hot milk mixture all at once, then whisk until smooth.
Step #6 Stir in spinach; cook this until sauce is thick & bubbly & spinach is tender but still green, about 5 mins.
Step #7 Stir in cheese, nutmeg, & a generous sprinkling of salt & pepper.