1-Getting started with this cheesy potato gratin casserole: is as easy as turning on your oven. First, preheat it to 375 degrees Fahrenheit and grease a 2-quart square baking dish or an 8×12 baking dish with cooking spray, butter, or oil to prevent sticking. This sets the stage for even baking and a beautiful finish.

2-Next: arrange the sliced potatoes in three tight, slanted rows in the dish, leaving a slight space so they aren’t packed too tightly, and interweave the onion slices between and on top for added flavor. To make the cheese sauce, melt the butter over medium heat, then whisk in a little flour and slowly add the milk, alternating to avoid lumps until it thickens into a creamy consistency.

3-Lower the heat and stir in the shredded sharp cheddar cheese, garlic powder, salt, and pepper, tasting to adjust as needed. Pour this sauce evenly over the potatoes and onions, cover the dish with foil, and bake for 45 minutes. Remove the foil, sprinkle on the gruyere and Parmesan cheeses, and bake uncovered for another 30 to 45 minutes until the potatoes are tender and the top is golden brown. Finally, let it cool for a bit and garnish with fresh chopped parsley before serving.