Bake 30 min. or until heated through, uncovering after 20 min. About 10 min. before lasagna is done, melt butter in saucepan on medium heat. Add flour; cook and stir 1 min. Gradually whisk in milk; bring to boil, stirring constantly. Simmer on medium-low heat 3 min. or until thickened, stirring constantly. Remove from heat; stir in remaining nutmeg.