1-First Step: Prep the potatoes and oven Start by preheating the oven to 350 degrees F. Peel the 2 pounds of Yukon Gold potatoes, then slice them into 1/8 inch thick rounds. Try to keep the slices uniform so they cook evenly. A mandoline works well here, but a sharp knife is fine if you take your time. If you want to save a little prep time later, you can also measure out the cheese, chop the onion, and mince the garlic before you begin the sauce. Having everything ready makes the cooking process calmer, especially when you are cooking for a holiday or a busy weeknight.

2-Second Step: Cook the onion and garlic Place a saucepan over medium-high heat and melt the 3 tablespoons butter. Once the butter is hot, add the 1/2 cup finely chopped yellow onion and cook until soft. This usually takes a few minutes and helps build the savory base of the sauce. Next, stir in the 2 cloves garlic, minced, and cook for another minute. Be careful not to brown the garlic too much. You want it fragrant and mellow, not bitter.

3-Third Step: Make the cream sauce Sprinkle in the 3 tablespoons all-purpose flour and stir until everything is well combined. Let it cook for a minute or two so the raw flour taste cooks off. This step is important because it helps the sauce thicken later. Slowly pour in the 2 cups whole milk and 1/2 cup heavy cream, stirring constantly to keep clumps from forming. Once the mixture is smooth, bring it to a boil, then reduce the heat and simmer until it thickens slightly. It should coat the back of a spoon, not turn into a paste. Take your time with the sauce. A smooth base makes the whole casserole taste richer and more polished.

4-Fourth Step: Add the cheese and seasonings Stir in the 1 1/2 cups shredded cheddar cheese, half of the 1/3 cup grated Parmesan cheese, the 1 1/2 teaspoons salt, and the 1/4 teaspoon freshly ground black or white pepper if using. Keep stirring until the cheese melts fully and the sauce looks glossy and creamy. This is the point where the kitchen starts smelling amazing. The cheddar gives the sauce that classic sharp flavor, while the Parmesan adds saltiness and a little nuttiness. If you want a stronger bite, you can use white pepper for a softer look and gentle heat, or black pepper for a more familiar finish.

5-Fifth Step: Combine the potatoes and sauce Add the sliced potatoes to the saucepan and stir until all the rounds are coated. The mixture will look thick, and that is exactly what you want. Every slice should get a good amount of sauce so the final casserole bakes up creamy from top to bottom. Once combined, pour everything into a 2-quart casserole dish. Spread the mixture out so it sits in an even layer. Sprinkle the remaining Parmesan over the top for a nicely browned finish after baking.

6-Final Step: Bake, rest, and serve Cover the casserole with foil and bake for 1 hour, or until the potatoes are tender. Then uncover the dish and bake for another 20 minutes so the top turns lightly browned. If your oven runs hot, check during the final stretch to keep the cheese from over-darkening. When the potatoes are done, let the dish sit for 5 minutes before serving. This short rest helps the sauce settle and gives the casserole a better texture. Finish with chopped parsley for a fresh look and serve warm.