Position a rack in the center of the oven, then place a 10-inch iron skillet or other 10-inch oven-proof skillet in the oven, and preheat to 425°F.
Meanwhile, prepare the topping: Combine the brown sugar, walnuts, flour, cinnamon and salt and a small bowl and whisk to blend. Add the butter and press the flour mixture and butter together between fingertips until mixture resembles a very coarse meal.
Prepare the cornbread: Combine the cornmeal, flour, baking powder, baking soda and salt in a medium bowl and whisk to blend. Combine the milk, yogurt, egg and brown sugar in another medium bowl and whisk to blend.
Using 2 potholders, remove the skillet from the oven and place on the stovetop or other heatproof surface. Immediately add the butter and stir until melted. Add the apples and stir for about a minute to coat and warm through. Using 2 potholders, pour the apples and melted butter into the bowl with the liquid ingredients, leaving enough butter in the skillet to coat. Mix apples and butter into the liquid ingredients and then pour the mixture into the dry ingredients. Fold together just until blended. Immediately pour the mixture into the hot skillet, using a rubber spatula to help. The mixture will start to rise. Sprinkle the topping over.
Using 2 potholders, place the skillet in the oven. Bake until the top of the cornbread is brown and feels firm to the touch and a tester inserted in the center comes out clean, 20 to 25 minutes. Using 2 potholders, transfer the skillet to a cooling rack. Let the cornbread cool at least 30 minutes before serving. Serve warm or at room temperature, with butter if desired.