Bring a medium saucepan of water to a boil; add the pasta shells and cook until al dente. Drain the pasta, rinse under cold running water and drain in a colander.
Meanwhile, in a large saucepan, melt the butter over medium heat; add the onion, celery and thyme and cook, stirring occasionally, until the onion and celery are softened, about 7 minutes. Add the flour and cook, stirring continuously, for 2 minutes. Whisk in the fish stock and bring to a boil, whisking occasionally.
Stir together the milk and cornstarch and then whisk the mixture into the fish stock mixture. Bring the soup to a boil, then cook, whisking continuously, for 2 minutes. Stir in the shrimp, scallops and peas. Cook the stew, stirring frequently, until the seafood is firm and just cooked through, about 5 minutes. Stir in the tomatoes, sherry and pasta shells; season to taste with salt and pepper and serve.
