1 easy zucchini gratin Recipes

unsalted butter, plus extra for topping (start with 3, then see how much more you want to put in), yellow onions, cut in 1/2 and sliced, garlic cloves, minced (optional), zucchini, sliced 1/4-inch thick (about 4 zucchini), kosher salt (or sea salt to taste), fresh ground black pepper, ground nutmeg, all-purpose flour, hot milk, panko breadcrumbs (or italian breadcrumbs), grated gruyere cheese (or gouda, cheddar, swiss or parmesan)