Preheat oven to 375°F (190°C). Grease a baking dish with butter or cooking spray.
2️⃣ Make the sauce: In a saucepan over medium heat, melt butter. Add garlic and cook for 30 seconds until fragrant. Stir in flour and cook for 1 minute, stirring constantly.
3️⃣ Whisk in liquids: Gradually add milk and heavy cream, stirring frequently until thickened (about 3-4 minutes). Add salt, pepper, and onion powder. Remove from heat.
4️⃣ Layer the potatoes: Arrange half of the potatoes in an even layer in the baking dish. Pour half the sauce over them, then sprinkle half the cheddar and Parmesan cheese on top.
5️⃣ Repeat layers: Add the remaining potatoes, followed by the remaining sauce and cheese.
6️⃣ Bake: Cover with foil and bake for 40 minutes. Remove foil and bake for 20-25 minutes, or until the top is golden brown and potatoes are tender.
7️⃣ Serve: Let rest for 10 minutes before serving. Garnish with fresh parsley if desired.