Saute mushrooms in olive oil. When mushrooms are browned, add onions and saute until just softened.

Meanwhile, melt butter in heavy saucepan over medium heat. Add garlic and lemon peel. Cook until aromatic, stirring occasionally, about 5 minutes. Gradually mix wine into flour and add to butter mixture. Season with salt and pepper. Stir until sauce thickens, about 1 minute.

Mix in tuna. Add basil, parsley and mushroom mixture and optional corn kernels. Place shells in serving bowl. Pour sauce over and toss. Sprinkle with fresh parsley. Serve hot.