Heat oven to 400 degrees. Add oil to a skillet and place in the oven to heat up.
Peel and slice the potatoes very thin. Set aside.
In a small bowl mix the mustard and mayonnaise. Set aside.
Remove the skillet from the oven and carefully cover the bottom with a single layer of potatoes, overlapping the edges slightly. Season with salt, pepper and thyme.
Pat dry the Tilapia and then brush the top and bottom with the mustard & mayonnaise mixture.
Place the Tilapia over the potatoes in the skillet, then arrange a second layer of potatoes over the fish, covering it entirely. Season with salt, pepper and thyme, then mist with cooking spray.
Place the skillet back into the oven and bake for 14 minutes. Increase the oven to broil and cook for another 2 or 3 minutes or until the potatoes are nicely browned. Let sit for a few minutes before serving.