Boil whole peeled potatoes to al dente. Drain, cool and cut to 1/4 inch slices. Line a 12" cast iron pan with the slices. Set aside.

In a sauce pan, caramelize the onions with the butter. Once caramelized, add the flour and continue cooking for a few minutes.

Add milk and bring to simmer.

Combine remaining ingredients (except parmesan cheese) into sauce pan and blend with immersion blender until smooth.

Cover cheese / potato mixture with the parmesan cheese.

Pour over potatoes and bake at 350 for 45 minutes until brown.

Allow to cool for at least half an hour before serving.