1-Getting started with this recipe: preheating your oven. Set it to 375 degrees F and grease a 2-quart square or 8×12 baking dish to avoid any sticking. This simple step sets the stage for a smooth cooking experience.

2-Next, arrange the potato slices in three tight slanted rows, adding the onion slices in between and on top. This layering helps the flavors meld together nicely. Then, for the cheese sauce, melt the butter over medium heat and gradually whisk in the flour while alternating with the milk to keep it smooth.

3-Bring the mixture to a simmer until it thickens, then stir in the sharp cheddar cheese, garlic powder, salt, and pepper. Taste and adjust as needed before pouring it evenly over the potatoes. Cover with foil and bake for 45 minutes, then remove the foil and add Gruyere and Parmesan on top.

4-Bake uncovered for another 30-45 minutes until the potatoes are tender and the top is golden. Finally, let it rest for a bit and garnish with fresh chopped parsley. For more tips on baking, consider visiting a reliable guide on potato dishes to refine your technique.