MELT butter in large saucepan over low heat. Add radicchio; cook, stirring occasionally, for 8 to 10 minutes.
PREPARE pasta according to package directions, reserving ½ cup cooking water.
ADD reserved cooking water and cheese to pan with radicchio. Cook, stirring occasionally, until cheese is melted and creamy. Add pasta; stir gently to coat. Stir in fresh thyme leaves, if using. Top with walnuts.