Bring a large pot of salted water to a boil. Add elbow macaroni and cook until al dente, about 7-8 minutes. Drain pasta and set aside.

In a blender, combine 4 cups of rinsed baby spinach with ½ cup of milk and blend until smooth.

In a medium saucepan, melt ¼ cup unsalted butter over medium heat. Stir in 3 tablespoons of flour, ¾ teaspoon of garlic, and ⅛ teaspoon of black pepper, mixing until bubbling.

Gradually add 1½ cups of milk and the spinach mixture, stirring constantly. Bring to a gentle boil, reduce heat, and simmer for about 1 minute until thickened.

Add 2 ⅔ cups of white cheddar and ⅔ cup of grated cheese, stirring until melted. Season with salt to taste.

Fold the drained macaroni into the cheese sauce until well-coated. Serve hot.