Sauté onion and green pepper in butter in a medium skillet until tender. Add flour and salt; stir well. Gradually add half-and- half, stirring until smooth. Add nutmeg and mustard; stir well. Gradually add egg yolks, stirring constantly. Stir in crabmeat; cook over medium heat about 5 minutes. Serve in individual baking shells or dishes, garnishing each with mushrooms and parsley, if desired.