Cook pasta as directed, drain. In lg pot, over medium heat, melt butter. Add flour, cook, stirring for 2 min. Whisk in milk. Cook, stirring until slightly thickened. About 5 min. Whisk in the mustard and nutmeg, 1 cup cheddar cheese, 1/2 tsp salt, and 1/4 tsp pepper. Add the pasta and spinch. Toss to combine. Heat broiler. Spray a 1-1/2 cup baking dish, or 4 12-oz ramekins w/ cooking spray. Transfer pasta mix to baking dish or ramekins. Sprinkle w/ remaining cheddar and broil until golden brown, about 3-4 min.