Preheat oven to 375 F. Prepare spinach according to package directions. In a medium saucepan over medium heat, melt butter. Cook garlic until fragrant. Whisk in flour and cook for a minute to brown.
Pour in milk, grated parmesan cheese, spinach, salt and pepper. Cook for a few minutes until combined.
Spread mixture into a baking dish. Top with shredded cheddar cheese and bake foe 30 minutes.