1-First Step: Prep the potatoes and oven Start by heating your oven to 350°F. Lightly grease a baking dish so the sauce does not stick. Peel the potatoes, then slice them into 1/8 inch thick rounds. Try to keep the slices even so they cook at the same pace. If you want to save a little time later, measure out all the ingredients before you begin cooking.

2-Second Step: Build the sauce base Set a medium saucepan over medium heat. Melt the 3 tablespoons of butter, then add the finely chopped yellow onion. Cook for about 3 to 4 minutes until the onion softens. Add the minced garlic and cook for 30 seconds more, just until fragrant. Stir in the 3 tablespoons of all-purpose flour and keep whisking for about 1 minute so the raw flour taste cooks off.

3-Third Step: Add the milk and cream Slowly pour in the 2 cups of whole milk while whisking. Add the 1/2 cup of heavy cream, then keep stirring until the mixture starts to thicken. This usually takes about 3 to 5 minutes. Season with 1 1/2 teaspoons salt and the 1/4 teaspoon freshly ground black or white pepper if you want a little extra warmth. The sauce should look smooth and lightly thick, not pasty.

4-Fourth Step: Stir in the cheese Take the pan off the heat. Stir in 1 cup of the shredded cheddar cheese and 1/6 cup of the grated Parmesan cheese. Save the remaining cheese for layering and finishing. The sauce should turn creamy and glossy. If it seems too thick, add a splash of milk. If it seems too thin, give it another minute or two over low heat.

5-Fifth Step: Layer the potatoes and sauce Place a thin layer of potatoes in the greased baking dish. Spoon some of the cheese sauce over the potatoes, then repeat the layers until you use everything up. Try to end with sauce on top so the potatoes stay moist during baking. Sprinkle part of the remaining cheddar and Parmesan between the layers for better flavor throughout the dish.

6-Sixth Step: Bake until tender Cover the dish with foil and bake for about 45 minutes. This helps the potatoes steam and soften. Then remove the foil, add the rest of the cheddar and Parmesan on top, and bake uncovered for 20 to 30 minutes more. The top should be bubbly and golden brown, and the potatoes should be easy to pierce with a knife.

7-Final Step: Rest, garnish, and serve Let the Au Gratin Potatoes rest for 10 to 15 minutes before serving. This helps the sauce set up a little, so the layers hold together better on the plate. Finish with chopped parsley for a fresh pop of color. Serve warm and enjoy the creamy, cheesy result.