1-First Steps: Preheat and Prep: Begin by preheating your oven to 375 degrees Fahrenheit and greasing a 2-quart square or 8×12-inch baking dish with nonstick spray, butter, or oil. This prep step takes about 30 minutes total and sets the stage for even cooking. Slice your potatoes into 1/8 inch rounds and prepare the onions to layer them just right.

2-Arranging the Layers: Arrange the sliced potatoes in three tight, slightly slanted rows in the baking dish, making sure not to pack them too tightly for the sauce to flow through. Insert the onion slices between the rows and on top for added flavor. This layering technique helps distribute the tastes evenly throughout the casserole.

3-Making the Cheese Sauce: Melt 2 tablespoons of salted butter in a pan over medium heat, then gradually whisk in ¼ cup of all-purpose flour while alternating with 1 ½ cups of unsweetened almond milk. Stir vigorously to avoid lumps until it thickens to a creamy consistency. Reduce the heat to low and mix in 8 ounces of sharp cheddar cheese, ½ teaspoon of garlic powder, ¾ teaspoon of salt, and freshly ground black pepper to taste.

4-Baking to Perfection: Pour the cheese sauce evenly over the potatoes and onions to coat them thoroughly, then cover the dish with foil and bake for 45 minutes. Remove the foil and sprinkle on ½ cup of Gruyere cheese and ¼ cup of grated Parmesan cheese before baking uncovered for another 30 to 45 minutes. The result should be a golden-brown top with fork-tender potatoes.

5-Finishing Touches: Let the casserole cool slightly, then garnish with fresh chopped parsley before serving. This step ensures the flavors settle and makes for easier slicing. Remember, you can adapt this for dietary needs, like using gluten-free flour in the sauce.