1-First Step: Prep the potatoes and oven Preheat your oven to 350 degrees F. Peel the Yukon Gold 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 a great time to use it, but a sharp knife works too.

2-Second Step: Cook the onion and garlic Set a saucepan over medium heat and melt the 3 tablespoons of butter. Add the 1/2 cup finely chopped yellow onion and cook for about 5 to 6 minutes, until soft and fragrant. Stir in the 2 cloves minced garlic and cook for 1 minute more. This builds a flavorful base for the sauce.

3-Third Step: Make the cheese sauce Sprinkle in the 3 tablespoons all-purpose flour and cook for 1 to 2 minutes, stirring often. This helps remove the raw flour taste and starts the thickening process. Slowly stir in the 2 cups whole milk and 1/2 cup heavy cream. Bring the mixture to a boil, then lower the heat and simmer until slightly thickened. Once the sauce has thickened, add the 1 1/2 cups shredded cheddar cheese, half of the 1/3 cup grated Parmesan cheese, 1 1/2 teaspoons salt, and 1/4 teaspoon ground black or white pepper if using. Stir until the cheese melts and the sauce looks smooth and creamy.

4-Fourth Step: Coat the potatoes Add the sliced potatoes to the sauce and stir gently until everything is fully combined. Make sure the potatoes are well coated so they bake evenly and absorb the flavor. If you want a slightly looser texture, you can add a small splash of milk, but avoid making the sauce too thin.

5-Fifth Step: Assemble the casserole Pour the potato mixture into a 2-quart casserole dish. Sprinkle the remaining Parmesan cheese over the top. Cover the dish tightly with foil so the potatoes steam and soften during baking.

6-Sixth Step: Bake until tender Bake the covered casserole for 1 hour, or until the potatoes are tender when pierced with a fork. If your slices are a little thicker, you may need a few extra minutes. The covered bake helps the potatoes absorb the sauce while staying moist.

7-Final Step: Brown and rest before serving Remove the foil and bake for 20 more minutes until the top is lightly browned. Let the dish sit for 5 minutes before serving so the sauce can settle. Finish with chopped parsley if desired. This final rest also helps the au gratin potatoes hold together when scooped.