1-First Step: Prep the potatoes and oven Preheat the oven to 350°F. Peel the Yukon Gold potatoes and slice them into 1/8-inch thick rounds. Try to keep the slices even so they bake at the same rate. If you are making this dish ahead, slice the potatoes and keep them in cold water for a short time, then dry them well before using.

2-Second Step: Start the onion base Place a pan over medium-high heat and melt the butter. Add the finely chopped yellow onion and cook for about 5 to 6 minutes, stirring often, until it turns soft and fragrant. This step gives the sauce a sweeter, fuller taste. If you prefer a stronger onion flavor, let the onion cook until just lightly golden.

3-Third Step: Add the garlic and flour Stir in the minced garlic and cook for 1 minute. Garlic cooks fast, so keep an eye on it and do not let it burn. Next, add the flour and cook for 1 to 2 minutes. This helps remove the raw flour taste and gives the sauce a smooth base for the milk and cream.

4-Fourth Step: Build the creamy sauce Slowly pour in the whole milk and heavy cream while stirring constantly. Add them gradually so the mixture stays smooth and does not clump. Bring the sauce to a boil, then lower the heat and let it simmer until slightly thickened. You want it rich enough to coat the potatoes, but still loose enough to bake properly in the oven.

5-Fifth Step: Stir in the cheese Add the cheddar cheese, half of the Parmesan cheese, the salt, and the optional black or white pepper. Stir until the cheese melts completely. The sauce should look glossy and smell wonderfully cheesy. If you are using Gruyere for a variation, this is the stage where it can melt in beautifully.

6-Sixth Step: Coat the potatoes Add the sliced potatoes directly into the sauce and stir until every piece is coated. This helps the potatoes bake up evenly and soak in flavor from the beginning. If the sauce looks very thick, a splash of extra milk can help loosen it. This is also a good point to taste the sauce and adjust the seasoning if needed.

7-Seventh Step: Assemble the casserole Transfer the potato mixture into a 2-quart casserole dish. Spread it out in an even layer so the potatoes bake evenly and the top browns nicely. Sprinkle the remaining Parmesan over the surface. That last layer of cheese helps create a lightly crisp, savory finish.

8-Eighth Step: Bake covered, then uncover Cover the dish tightly with foil and bake for 1 hour, or until the potatoes are tender when pierced with a knife. After that, remove the foil and bake for 20 more minutes. This final stretch allows excess moisture to evaporate and gives the top a lightly browned look.

9-Ninth Step: Rest and serve Take the dish out of the oven and let it rest for 5 minutes before serving. This short rest gives the sauce time to settle so the portions hold together better. Finish with chopped parsley for a fresh pop of color and serve while warm.