1-First Step: Get the oven and pan ready Preheat your oven to 375 degrees F. Then grease a 9×9-inch oven-safe baking pan or a similar equivalent dish. This helps keep the casserole from sticking and makes serving much easier later. While the oven heats, go ahead and slice the zucchini and onion if you have not done that yet. For the best texture, slice both the zucchini and onion about 1/4 inch thick. Thin enough to soften, but not so thin that they turn to mush. That little bit of structure gives the casserole a nicer bite when it comes out of the oven.

2-Second Step: Build the first layer Overlap about one-third of the zucchini and onion slices in the prepared baking pan. Try to spread them out evenly so each bite gets a little of both. Season this first layer with salt and pepper, then sprinkle on 1/2 cup of the shredded pepper jack cheese. This layering approach helps the cheese melt between the vegetables, which means better flavor all the way through. No sad, dry middle layers here.

3-Third Step: Repeat the layering Repeat the same process two more times until you have used up all of the zucchini, onions, and shredded cheese. You should end up with three good layers total. Keep the layers even and slightly overlapping so everything cooks at the same pace. If some of the slices are a little uneven, no big deal. This is home cooking, not a geometry quiz.

4-Fourth Step: Make the creamy sauce In a microwave-safe dish, combine the garlic powder, butter, heavy whipping cream, and xanthan gum. Heat the mixture for about one minute, or until the butter has melted. Then whisk until smooth. The xanthan gum helps thicken the sauce, so whisking matters here. If you skip that step, you may end up with little clumps, and nobody wants surprise sauce lumps in their gratin.

5-Fifth Step: Pour and bake Gently pour the butter and cream mixture over the layered zucchini and onions. Try to coat the top evenly so the sauce can work its way down through the casserole as it bakes. Place the dish in the oven and bake at 375 degrees F for about 45 minutes. The casserole is ready when the liquid has thickened and the top is golden brown. If your oven runs hot, start checking around the 40-minute mark. If it still looks a little loose, that is okay. It should settle as it cools slightly.

6-Final Step: Serve warm and enjoy Let the casserole sit for a few minutes before serving. That short rest helps the sauce thicken up a bit more and makes it easier to cut clean squares. Serve warm as a side dish with chicken, steak, pork, or even a simple salad for a lighter meal.