Follow these simple steps to create your own creamy and cheesy au gratin masterpiece:

1. Preheat the Oven: Set your oven to 375°F (190°C) and grease a 2-quart baking dish to prevent sticking.
2. Prepare the Potatoes and Onions: Layer the thinly sliced potatoes in the baking dish, inserting onion slices between the layers to infuse flavor throughout.
3. Make the Sauce: In a saucepan, melt the butter over medium heat. Gradually whisk in the flour to create a roux. Slowly add the almond milk, continuing to whisk until the mixture is smooth and creamy. Bring to a gentle simmer.
4. Add Cheese and Seasoning: Lower the heat and stir in the cheddar cheese, garlic powder, salt, and black pepper. Taste the sauce and adjust the seasoning if necessary.
5. Assemble the Dish: Pour the cheese sauce evenly over the layered potatoes and onions. Cover the dish with foil to help the potatoes cook evenly.
6. Bake: Place the dish in the oven and bake for 45 minutes. This initial baking time allows the potatoes to soften and absorb the cheesy flavor.
7. Add More Cheese and Finish Baking: Remove the foil and sprinkle the Gruyère (or additional cheddar) and Parmesan cheese on top. Return the dish to the oven, uncovered, and bake for another 30-45 minutes until the top is golden brown and the potatoes are tender.
8. Garnish and Serve: Once out of the oven, garnish with freshly chopped parsley for a touch of color and freshness. Serve hot and enjoy the creamy, cheesy goodness.