Stir in apples and walnuts, batter will be thick. Spread batter into prepared pan.
Sprinkle the streusel mixture over the coffee cake batter in the pan. Set aside.
Bake the coffee cake for 50-60 minutes, or until the top is light golden brown and the cake feels firm to the touch, or a toothpick comes out clean. Let cool in the pan on a wire rack for 20 minutes. Run a butter knife around the cake’s edges to loosen it, then unmold.