3. In a bowl, mix the eggs, cream, milk, a little salt, pepper, and a quarter of the gruyère cheese. Unroll the pastry, place it in a pie dish with its parchment paper, and prick the bottom with a fork. Spread the onions over the pastry, add half of the gruyère cheese, and pour the egg mixture over it. Finish by sprinkling the remaining gruyère cheese on top.