Baked Apples with Candied Walnuts

Put oven rack in middle position and preheat oven to 450 °F. Oil a baking sheet. Halve apples lengthwise and remove core with a melon-ball cutter or a paring knife. Melt butter in a 10-inch heavy ovenproof skillet over moderate heat and sprinkle with 1/2 tablespoon sugar. Arrange apple halves, cut sides down, on sugar and cook, undisturbed, until sugar begins to caramelize, 3 to 5 minutes. Transfer skillet to oven and bake apples until tender, about 15 minutes. While apples are baking, cook remaining 1/4 cup sugar with salt in a dry 7- to 8-inch heavy skillet over moderately high heat, undisturbed, until sugar begins to melt, about 2 minutes. Continue to cook, stirring occasionally with a fork, until sugar is melted into a deep golden caramel, 1 to 2 minutes more. Remove from heat. Add walnuts, stirring to coat, then pour onto baking sheet and cool 10 minutes. Break nuts into small pieces if necessary. Stir together yogurt and 2 teaspoons honey. Serve apples, cut sides up, topped with yogurt and nuts. Drizzle with additional honey if desired.