Step 1: Preheat your oven to 375°F (190°C). Lightly butter a 9x13 inch (or similar size) baking dish.
Step 2: In a large saucepan, melt 2 tablespoons of butter over medium heat. Add the thinly sliced Vidalia onions and cook, stirring occasionally, for 10-15 minutes, or until softened and translucent. Season lightly with 1/2 teaspoon salt and 1/4 teaspoon black pepper.
Step 3: While the onions are softening, whisk together the heavy cream, chicken or vegetable broth, minced garlic, chopped fresh thyme, finely chopped fresh rosemary, remaining 1/2 teaspoon salt, and 1/4 teaspoon black pepper in a medium bowl. Add the optional nutmeg if using.
Step 4: Arrange about one-third of the softened Vidalia onions in an even layer in the prepared baking dish. Sprinkle with about one-third of the shredded Gruyère and Parmesan cheeses. Pour about one-third of the cream mixture over the onions and cheese.
Step 5: Repeat the layering process two more times, ensuring each layer of onions is evenly distributed and finishing with a generous layer of the remaining Gruyère and Parmesan cheeses on top.
Step 6: Cover the baking dish tightly with aluminum foil. Bake for 30 minutes in the preheated oven.
Step 7: Remove the foil and continue to bake for another 20-30 minutes, or until the top is golden brown and bubbly, and the onions are fork-tender. If the cheese begins to brown too quickly, you can loosely tent the dish with foil. Let the gratin rest for 5-10 minutes before serving. Serve warm.