1-First Step: Prepare the potatoes and oven Preheat the oven to 350°F. Peel the Yukon Gold potatoes and slice them into 1/8-inch rounds. A mandoline works especially well for this because the slices come out even, which helps the potatoes cook at the same rate. If the slices are too thick, they may stay firm in the center while the sauce finishes cooking. If you are cooking for a crowd or trying to save time, you can slice the potatoes earlier in the day and keep them in cold water. Just drain and pat them dry before cooking so the sauce stays creamy and not watery.

2-Second Step: Cook the onion and garlic Set a pan over medium-high heat and melt the butter. Add the finely chopped yellow onion and cook for about 5 to 6 minutes, or until soft and translucent. This step builds a sweet base that keeps the sauce from tasting flat. Add the minced garlic and cook for another minute. Stir often so the garlic does not brown too quickly. You want a gentle garlic flavor that blends into the sauce instead of overpowering it.

3-Third Step: Make the creamy cheese sauce Stir in the flour and cook for 1 to 2 minutes. This helps remove the raw flour taste and gives the sauce structure. Next, slowly add the whole milk and heavy cream, stirring constantly so lumps do not form. Keep the heat steady and patient here. A smooth sauce is what makes this dish feel rich and velvety. Bring the mixture to a slow boil, then lower the heat and simmer until it thickens slightly. The sauce should coat the back of a spoon. It does not need to be super thick at this stage because it will continue to thicken in the oven.

4-Fourth Step: Add the cheese and seasonings Stir in the cheddar cheese, half of the Parmesan cheese, salt, and pepper if you are using it. Keep stirring until the cheese melts fully. Do not use pre-shredded cheddar here if you can avoid it, because it often contains anti-caking agents that stop it from melting as smoothly. The sauce should now taste savory, creamy, and just a little sharp from the cheese. If you like a stronger pepper note, use freshly ground black pepper. If you prefer a softer look, white pepper works too.

5-Fifth Step: Coat the potatoes Add the sliced potatoes to the pan and stir carefully until every piece is coated in the sauce. Try to lift and fold instead of stirring aggressively so the slices stay intact. Every potato round should be covered, because that is what helps the final dish bake evenly and stay moist. If you are making this for a holiday meal, this is a great stopping point for a make-ahead strategy. You can assemble the dish earlier in the day, cover it, and refrigerate it until you are ready to bake.

6-Sixth Step: Assemble and bake Transfer the potato mixture to a 2-quart casserole dish. Sprinkle the remaining Parmesan cheese over the top, then cover the dish with foil. This first covered bake lets the potatoes soften without drying out. Bake for 1 hour, or until the potatoes are tender when pierced with a fork. Then uncover the dish and bake for another 20 minutes. During this final stretch, the top should turn lightly browned and any extra moisture should evaporate. If you want a slightly deeper top, leave it in the oven just a few minutes longer, but keep an eye on it.

7-Final Step: Rest and serve Let the casserole rest for 5 minutes before serving. This short wait helps the sauce settle so the potatoes hold together better when scooped. Finish with chopped parsley for a fresh pop of color.