1. Preheat the oven to 350°F.
2. In a saucepan, melt the butter and sauté the onions.
3. Deglaze with white wine. Season with salt and pepper. Add parsley.
4. Bring to a boil and add the clams. Remove them as soon as they open. Taste.
5. Remove the clamshells and arrange the clams in a baking dish.
6. Sprinkle the clams with grated gruyere cheese. Bake for 4 minutes.
7. Remove from the oven and serve.