Cut 8 large tomatoes in half, scoop out the centers, sprinkle the interiors with salt and a pinch of sugar, and fill with this cheese mixture. Top the filling with a bit of extra Gruyère. Arrange in a baking dish, add a bit of water to the dish, cover, and bake in a 350 degrees F oven until the tomatoes are tender, 25 to 45 minutes, depending on their size. Serve hot.