- Preheat oven to 400ºF and lightly grease a 9×13 oven safe dish with cooking oil. Peel potatoes thinly using a sharp knife or mandolin. Set them aside.
- Heat butter over medium-low heat in a saucepan. Once butter is melted, whisk in flour. Heat for 1 minute. Gradually whisk in milk. Add sriracha, garlic powder, black pepper and salt. Bring mixture to a simmer and cook for 6-8 minutes until thickened.
- Turn off the heat and stir in ½ cup of Gouda cheese and ½ cup of cheddar cheese until smooth.
- Spread half of the sliced potatoes evenly in prepared dish. Pour half of the cheese sauce over potatoes. Layer the remaining potatoes on top and pour the remaining cheese sauce over the top.
- Sprinkle the remaining Gouda and white cheddar cheese on top and cover with aluminum foil.
- Bake for 25 minutes. Remove aluminum foil and continue to bake for 20-25 minutes longer, until potatoes are tender and the top is golden brown.
- Cool at room temperature for 10 minutes before serving to allow cheese sauce to set. Serve garnished with fresh thyme.