Spread each slice of the bread with 1 tablespoon of the butter and set on a nonstick baking sheet. Fan the apple slices in an overlapping pattern over the bread; drizzle with any remaining juices. Dot with the remaining 1 tablespoon of butter and top with the walnuts. Bake the tartines on the bottom shelf of the oven for 15 minutes, or until the apples are tender and the bread is toasted on the bottom. Sift confectioners' sugar over the tartines and serve.