1-First Step: Prep the potatoes and baking dish Start by preheating your oven to 350°F. Butter a medium baking dish or a 2 to 2 1/2 quart casserole dish. Peel the potatoes and slice them into 1/8 inch thick rounds. Try to keep the slices even so they cook at the same pace. If you have a mandoline, this is the perfect time to use it. Set the sliced potatoes aside in a bowl of cold water if you are not moving right into assembly, but drain and pat them dry before layering.

2-Second Step: Build the sauce base Set a medium saucepan over medium heat and melt the butter. Add the chopped onion and cook for 3 to 4 minutes, stirring now and then, until it softens. Stir in the minced garlic and cook for about 30 seconds, just until fragrant. Sprinkle in the flour and whisk for about 1 minute to form a smooth roux. This step helps the sauce thicken later, so do not rush it.

3-Third Step: Add the milk and cream Slowly pour in the whole milk while whisking so the sauce stays smooth. Add the heavy cream next. Keep whisking until the mixture starts to thicken, which usually takes 4 to 6 minutes. The sauce should coat the back of a spoon. Add the salt and the black or white pepper if using. Taste the sauce and adjust lightly if needed.

4-Fourth Step: Stir in the cheese Take the pan off the heat and stir in 1 cup of the cheddar cheese and half of the Parmesan cheese. Keep stirring until the cheese melts into the sauce. The mixture should look creamy and glossy. If it seems too thick, add a small splash of milk. If it looks too thin, let it sit for a minute before assembling.

5-Fifth Step: Layer the potatoes and sauce Arrange a layer of potato slices in the buttered dish, overlapping slightly. Spoon some sauce over the top. Keep layering potatoes and sauce until everything is used up, finishing with sauce on top. Gently press down with a spatula so the sauce settles between the layers. This helps the potatoes bake evenly and stay creamy inside.

6-Sixth Step: Add the finishing cheese Sprinkle the remaining 1/2 cup cheddar and the rest of the Parmesan over the top. This gives the dish a golden, savory crust after baking. If you like a little extra color, a light dusting of pepper can go on top too.

7-Seventh Step: Bake until tender Cover the dish tightly with foil and bake for 45 minutes. Then remove the foil and bake for another 25 to 35 minutes, or until the potatoes are tender when pierced with a knife and the top is bubbling and browned. If the top browns too fast, loosely cover it again with foil.

8-Final Step: Rest and serve Let the dish rest for 10 to 15 minutes before serving. This helps the sauce settle and makes scooping easier. Finish with chopped parsley for a fresh pop of color. Serve warm with roast meats, chicken, or a crisp salad.