FOR THE APPLE TOPPING

3 apples (about 1 1/2 pounds), peeled, cored and sliced 1/4-inch thick

1/2 cup dark brown sugar

8 tablespoons (1 stick) unsalted butter

Pinch kosher salt

1/2 cup chopped walnuts, toasted (optional)

FOR THE MUFFINS

2 cups all-purpose flour

3/4 cup dark brown sugar

1 tablespoon baking powder

1/2 teaspoon ground cinnamon

1/4 teaspoon kosher salt

8 tablespoons (1 stick) unsalted butter, melted

2 large eggs, lightly beaten

3/4 cup sour cream

1 teaspoon vanilla extract.