Preheat oven to 400 degrees F (200 degrees C).
In a medium saucepan, add Butter (3 tablespoon) over medium-high heat and let it melt.
Once the butter is melted, add All-Purpose Flour (3 tablespoon). Mix together and add the Dried Thyme (1/2 teaspoon) and Onion (1). Cook for about 1 minute.
Combine Whole Milk (1 1/4 cup) and Heavy Cream (1/2 cup), and add both into the saucepan.
Add Salt (1 1/2 teaspoon) and Ground Black Pepper (to taste). Stir occasionally and let thicken.
Once mixture has thickened, add Sharp Cheddar Cheese (6 ounce) and allow it to melt.
Pour some of the sauce into the bottom of a baking dish, and spread for an even coating.
Layer half of the Potato (2 pound) on top, and top with another layer of sauce, again spreading to create an even layer.
Lay the last of the potatoes and top with the last of the sauce.
Sprinkle Sharp Cheddar Cheese (2 ounce) on top of the dish.
Cover with aluminum foil and bake for 45 minutes or 1 hour in the preheated oven.
Remove the foil and bake for another 15-20 minutes, until cheese is melted and bubbly.
Let cool. Serve and enjoy!