1-First Step: Get the oven and pan ready Start by preheating your oven to 375 degrees. Then grease a 9×9 inch oven-proof pan, or use an equivalent baking dish if that is what you have on hand. A well-greased dish helps the cheesy edges lift easier later, which matters when that crispy corner piece is the one everybody fights over. Before you start layering, make sure your zucchini is sliced into about 1/4 inch thick rounds. That thickness gives you a nice tender bite without turning the casserole into mush. If the slices are too thick, they can stay a little firm in the center. If they are too thin, they may release too much water and get soft faster than you want.

2-Second Step: Build the zucchini gratin layers Take one-third of the sliced zucchini and one-third of the thinly sliced onion and overlap them in the bottom of the dish. Season that first layer with salt and pepper, then sprinkle on 1/2 cup of the shredded pepper jack cheese. This first layer is where the flavor starts building, so do not be shy with the seasoning. Repeat the same layering process two more times, using up all of the zucchini, onion, and cheese. The goal is to create even layers so every bite has a good mix of tender zucchini, onion, and melty cheese. This is what gives the casserole that classic au gratin look and feel.

3-Third Step: Mix the creamy sauce Now grab a microwave-safe dish and add 1/2 teaspoon garlic powder, 2 tablespoons butter, 1/2 cup heavy whipping cream, and 1/4 teaspoon xanthan gum. Microwave the mixture for one minute, just until the butter melts. Give it a good whisk until the sauce looks smooth and evenly blended. The xanthan gum helps the sauce thicken as it bakes, which is useful because zucchini naturally gives off moisture. That little bit of thickening power keeps the casserole from turning into a soupy situation. Nobody wants a watery gratin when they were promised cheesy comfort food.

4-Fourth Step: Pour and bake Pour the creamy mixture evenly over the layered zucchini and cheese. Try to get it into the corners so the sauce seeps through the whole casserole. Then place the pan in the oven and bake at 375 degrees for about 45 minutes. As it bakes, the sauce thickens, the cheese melts, and the top turns golden brown. Your kitchen will smell like a dream by the halfway mark, so fair warning: hungry people tend to wander in and ask when dinner is ready. If the casserole looks nicely browned and the liquid has thickened, it is probably right where you want it.

5-Final Step: Serve warm and enjoy Take the casserole out of the oven and let it sit for a few minutes before serving. That short rest helps everything settle a bit, which makes it easier to scoop neat portions. Serve it warm as a side dish with grilled meats, roast chicken, pork, or a simple weeknight main. This low carb zucchini gratin recipe is a good fit for family dinners, meal prep, or holiday meals when you need a veggie dish that does not feel like an afterthought. If you like serving rich, savory sides with barbecue plates, it pairs nicely with dishes like grilled pork chops or a juicy perfect ribeye steak.