Preheat your oven to 350°F (175°C).
In a medium saucepan, melt the butter over medium heat. Once melted, whisk in the flour and cook for about 1 minute until bubbly.
Gradually whisk in the heavy cream and milk. Continue to whisk until the mixture thickens (about 3-5 minutes).
Add the garlic powder, onion powder, and season with salt and pepper. Stir in 1½ cups of the shredded cheddar cheese until melted and combined.
In the baking dish, arrange half of the sliced potatoes in an even layer. Pour half of the cheese sauce over the potatoes.
Repeat the layers with the remaining potatoes and cheese sauce. Top the final layer of cheese sauce with the remaining ½ cup of shredded cheddar cheese.
Cover the baking dish with aluminum foil and bake for 30 minutes.
Remove the foil and bake for an additional 15 minutes until the potatoes are tender and the top is golden and bubbly.
Let it sit for about 5-10 minutes before serving. Garnish with fresh parsley if desired.