Au gratin potatoes with Salmon

Mix the following in a bowl. 

3 oz finely grated Gruyere cheese

2 ounces finely grated Parmigiano- Reggiano

2 cups heavy cream

2 medium garlic cloves, minced

1 T fresh thyme, roughly chopped

Kosher salt & black pepper

Add:

4-4 1/2 pound russet potatoes, sliced 1/8 inch on a mandoline 

2 T butter for the bottom of the pan

Add potato slices, covered in the cream sauce to a 9X13 buttered baking dish. Leave room on the side for the Salmon. Cover with foil and bake for 30 minutes. Remove and add the salmon, salt and pepper then smother with remaining cream mixture and bake additional 30 minutes, until potatoes are tender, and salmon is baked, about an hour total .