1You will need a 10 or 12 inch cast iron skillet.
Pre-heat oven to 400 degrees. Add oil to skillet and set in heated oven to heat.
2In a mixing bowl add mustard, mayonnaise, garlic, onion, and 1/2 teaspoon dried thyme. Mix together well.
Using a mandolin set to the thinnest setting. Slice potatoes and set a side.
3Remove skillet from oven. ( Be care full it will be hot).
Carefully put a layer of potatoes overlapping on the bottom of the skillet. Season potatoes with a little salt and pepper And thyme
4Pat the fish dry. Brush mustard-mayonnaise mix over both sides of fillets. Lay fish over potatoes. Place a second layer of potatoes over the top of the fish. Season with salt, pepper, and thyme. Spray top of potatoes with cooking spray..
5Place skillet in oven and bake for 14-16 minutes. after 16 minutes. Remove skillet, set oven to broil. Replace skillet and broil for 3-4 minutes or until potatoes are brown.
6Remove skillet from oven and set on pot holders.
Carefully divide into 4 pieces and set on plates.