Cook the Pasta: Bring a large pot of salted water to a boil. Add the pasta shells and cook according to the package instructions until al dente. Drain the pasta and rinse under cold water to stop the cooking process. Set aside.
Prepare the Ingredients: While the pasta is cooking, finely chop the shallots and celery. If using frozen peas, ensure they are thawed.
Combine Ingredients: In a large mixing bowl, combine the cooked pasta shells, flaked tuna, chopped shallots, chopped celery, and peas.
Add the Dressing: Add the mayonnaise to the bowl and mix well until all ingredients are evenly coated. Season with salt and pepper to taste. Adjust seasoning if needed.
Chill and Serve: Cover the bowl with plastic wrap and refrigerate for at least 1 hour to let the flavors marinate.