Cod and Shrimp Stoup with Mashed Potatoes

Name: Cod and Shrimp Stoup with Mashed Potatoes
Ingredients: 4 large baking potatoes ; peeled and thickly sliced; 3 tablespoons extra-virgin olive oil; 2 onions ; thinly sliced; 4 ribs celery, from the heart ; with leafy greens, chopped; 3 cloves garlic ; finely chopped; 1 large Bay leaf; 2 tablespoons fresh thyme leaves ; chopped; 1 Lemon ; grated peel and juice; 1/2 cup Dry white wine; 1 cup Chicken broth; 14 1/2 ounce diced or stewed tomatoes; 1 1/2 pounds cod fillet ; thick, cut into chunks; 1 pound large shrimp ; peeled and deveined; 4 tablespoons white balsamic vinegar or white wine vinegar; 2 tablespoons Butter