Creamy scalloped potatoes

Ingredients
3 tablespoons + 1 teaspoon butter, divided
3 tablespoons flour
1 teaspoon dry mustard
1 teaspoon kosher salt
2 cups milk
1/2 teaspoon freshly ground black pepper
1 tablespoon chopped fresh thyme leaves
2 pounds yukon gold potatoes, peeled and thinly sliced crosswise
1 medium onion, diced
Instructions
Preheat oven to 375 degrees.
Place a saucepan over medium heat and add butter. Using a whisk, swirl the butter around in the pan until it's almost melted. Add the flour and stir briskly to incorporate the flour and butter. Sprinkle in the mustard and salt and continue to cook for one minute over medium heat. Whisk in the milk and continue to stir, until the liquid comes to a rolling boil. Constantly stir and scrape the bottom of the pan while the sauce continues to cook and thicken, about 1 minute. Remove from heat and set aside.
Spray an 8x8" baking dish with vegetable spray. Spoon 2-3 tablespoons of the béchamel (milk mixture) into the bottom of the dish.
Sprinkle lightly with onions. Arrange potatoes in the bottom of the dish, overlapping as you go. Sprinkle with 1/3 of the onions. Pour 1/4 of the béchamel evenly over the potatoes. Continue layering in the same manner: Potatoes, onions, béchamel; potatoes, onions, béchamel. Finish with a layer of potatoes, topped by the last bit of béchamel.
Place baking dish onto a rimmed baking sheet. Spray a piece of tin foil on one side with vegetable spray. Tent the foil over the potatoes and place into the hot oven. Set timer for 45 minutes.
After timer goes off, reduce heat to 350 degrees and remove the tin foil. Continue to bake for an additional 45 minutes.
Cool for 5-10 minutes before serving.
Notes
this recipe can be doubled