Ahead of time, make a batch of salted caramel sauce. You can buy some from the store, but homemade is oh so much better.

Preheat oven to 350° F. Grease and/or line an 8x8 pan.

In bowl of stand mixer with paddle attachment on, cream butter and sugars until light and fluffy for several minutes. Scrape sides down and beat in egg. Scrape sides again.

In small bowl, combine the flour, baking soda, salt, cinnamon and nutmeg. Gradually add to creamed mixture and mix just until combined.

Toss apples in lemon zest and juice. Toast raw walnuts in small pan on stove over low heat, occasionally tossing walnuts to evenly heat. Add apples with lemon and walnuts to cake batter and set mixer on low just to get them going and then flash it on high for only a couple seconds to evenly mix them in.

Pour into prepared baking dish. Batter will be very thick so you will need to use a rubber spatula to spread it to an even layer. Bake in preheated oven for 30-35 minutes or until a toothpick inserted near the center comes out clean.

Serve cake warm or cold with warm caramel sauce with some salt crystals on top.