Caramelized onion mashed potatoes

My husband is a fan of mashed potatoes. He said these were the best he has ever eaten.

Ingredients
1 large yellow onion
6 medium potatoes
salt
3 tbsp butter
milk

Preparation
Slice the onion into 0.5-inch slices. Peel the potatoes and cut into quarters. Boil in salty water.

Preheat a cast iron skilled to hot (but never on maximum heat). Add a little bit of oil, add the sliced onions. Heat stirring occasionally until they caramelize. This works amazingly in heavy, cast iron skillets.

When the potatoes are soft, drain, mash with salt, butter, milk and caramelized onions. Serve.