Preheat oven to 375 degrees (F).
Grease a 9×9 or equivalent oven proof pan.
Overlap 1/3 of the zucchini and onion slices in the pan, then season with salt and pepper and sprinkle with 1/2 cup of shredded cheese.
Repeat two more times until you have three layers and have used up all of the zucchini, onions, and shredded cheese.
Combine the garlic powder, butter, and heavy cream in a microwave safe dish.
Heat for one minute or until the butter has melted. Stir.
Gently pour the butter and cream mixture over the zucchini layers.
Bake at 375 degrees (F) for about 45 minutes, or until the liquid has thickened and the top is golden brown.