Spread butter over 1 side of each bread slice. Arrange half of the bread slices overlapping in a greased 3-qt. or 13x9-in. baking dish, buttered side down. Spread marmalade over bread slices; top with remaining bread slices, buttered side up., In a large bowl, whisk eggs, milk, sugar, vanilla and nutmeg until blended; pour over bread. Refrigerate, covered, several hours or overnight., Preheat oven to 350°. Remove casserole from refrigerator while oven heats. Sprinkle with walnuts. Bake, uncovered, until golden brown and a knife inserted in the center comes out clean, 40-50 minutes., Let stand 5-10 minutes before serving. In a small saucepan, combine maple syrup, orange juice and zest; heat through. Serve with casserole.