In a deep pot, add the potatoes and enough water to cover. Bring to a boil, salt the water and cook the potatoes until tender, about 15 minutes. Drain.
In a Dutch oven or a large, deep skillet with a lid, heat the EVOO, 3 turns of the pan, over medium-high heat. Add the onions, celery, garlic, bay leaf, thyme and lemon peel; season with salt and pepper. Cook until the onions are softened, 7 to 8 minutes. Pour in the wine and cook for 1 minute. Stir in 1/2 cup chicken broth and the tomatoes with their juice; bring to a simmer. Add the cod in a single layer, cover and cook for 3 minutes. Gently stir in the shrimp; season with salt and pepper. Cover and cook until the cod and shrimp are just opaque throughout, about 3 minutes. Remove from the heat and stir in the lemon juice; discard the bay leaf.
Mash the potatoes with the remaining 1/2 cup chicken broth, the vinegar and butter; season with salt.
To serve, mound the potatoes into shallow bowls. Ladle the stoup around the potatoes.