1-Step 1: Prep the potatoes and oven Preheat your oven to 350°F, or 175°C. Peel the potatoes and slice them into 1/8-inch thick rounds for even cooking. If you want neat layers and faster prep, a mandoline helps a lot. Keep the slices similar in size so the dish bakes evenly from edge to center.

2-Step 2: Start the sauce base Place a large pan over medium-high heat and melt the butter. Add the chopped yellow onion and cook for 5 to 6 minutes, stirring often, until softened. Then add the minced garlic and cook for 1 more minute. This step builds the savory base that gives potatoes au gratin their signature flavor.

3-Step 3: Make the roux Stir in the all-purpose flour and cook for 1 to 2 minutes. You want the flour to cook a little, but not brown too much. This forms a roux, which helps thicken the sauce so it turns silky instead of runny. If you are using a gluten-free blend, cook it the same way and whisk well.

4-Step 4: Whisk in the milk and cream Slowly pour in the whole milk and heavy cream while whisking constantly. This is the part that keeps lumps from forming. Bring the mixture to a boil, then lower the heat and simmer until it thickens slightly. The sauce should coat the back of a spoon. If you prefer a lighter au gratin potato recipe, you can use a bit less cream and a bit more milk, but the sauce will be less rich.

5-Step 5: Add the cheese and seasonings Stir in the shredded cheddar cheese, half of the Parmesan, the salt, and the optional pepper. Keep stirring until everything melts into a smooth, creamy sauce. The cheddar gives the classic cheesy taste, while the Parmesan sharpens the flavor and helps the top brown nicely. This is the point where the dish starts smelling irresistible.

6-Step 6: Coat the potatoes Add the sliced potatoes to the sauce and gently mix until every slice is coated. Try not to break the potatoes too much, especially if you are using Yukon Gold. Moderate-starch potatoes are a smart pick because they stay firm enough to hold shape but still absorb that creamy sauce beautifully.

7-Step 7: Assemble the casserole Transfer the potato mixture to a greased 2-quart casserole dish. Spread it out evenly, then top with the remaining Parmesan. Cover the dish tightly with foil. This keeps the moisture in while the potatoes soften and the sauce gets absorbed.

8-Step 8: Bake until tender Bake covered for 1 hour, or until the potatoes are tender when pierced with a knife. Depending on slice thickness and your oven, the time may vary a little. If your potato slices are thicker, they may need a few extra minutes. If they are very thin, check early so they do not overcook.

9-Step 9: Finish uncovered for a golden top Remove the foil and bake for 20 more minutes. This uncovered bake lets excess liquid evaporate and helps the top turn golden and bubbly. If you want a deeper color, you can broil briefly at the end, but keep a close eye on it so the cheese does not burn.

10-Step 10: Rest and serve Let the dish rest for 5 minutes before serving. This short rest helps the sauce settle and makes serving easier. Sprinkle chopped parsley over the top if you like a fresh finish. The result is a creamy, comforting side dish that pairs well with roast chicken, ham, steak, or a simple green salad.