Preheat oven to 350°.
Peel, core and chop apples into bite-sized pieces.
Coarsely chop the walnuts.
In a large bowl, add vegetable oil, sugar, eggs and vanilla and mix with a wooden spoon.
Mix in apples and walnuts.
Add flour, baking soda, ginger, cinnamon and salt, stirring until combined.
Spray a 9 x 13 inch baking pan with non stick baking spray. Pour cake batter into pan, spreading evenly.
Bake for 50 minutes to one hour or until a toothpick inserted comes out clean.
Place 1/2 cup of chopped walnuts onto a baking sheet and place in the warm oven, with the oven turned off. Toast for 10 minutes. Remove from the oven and set aside.
Make the icing. In a heavy saucepan, add brown sugar, butter and heavy cream. Cook over medium heat, stirring until mixture comes to a rolling boil. Remove from heat.
Pour into a mixing bowl and set aside to cool about 10 minutes.
Add powdered sugar and vanilla.
Mix until well combined. Set aside until cake has cooled.
Spread icing over cooled cake.
Top with toasted walnuts.