Bring a large pot of salted water to a boil. Cook the shells according to package directions. Drain and reserve.
Heat a large pot over medium heat. Add the oil and sauté shallot, garlic, and thyme until aromatic, about 1 minute. Add the wine, fish stock, saffron and tomatoes to the pan and bring to a boil.
Add the mussels, cover and steam until the shells open, about 5 minutes. Discard any that don’t open.
Add the shrimp and fish and cook until cooked through, about 2 minutes. Taste and adjust seasoning with salt and pepper.