Open the cans of 14 ounces tuna, drain well. Boil the eggs and set aside to cool. Dice the 3 hard boiled eggs.
Dice the ½ cup red onion, halve the 1 cup cherry tomatoes and chop the 2 stalks celery.
Make the dressing
In a small mixing bowl, whisk together the 1 cup mayonnaise, 1 tablespoon lemon juice, and 2 teaspoons Dijon mustard until smooth.
Season with ½ teaspoon salt and ¼ teaspoons pepper to taste. Adjust the dressing according to your preference - if you like it tangier, add more lemon juice.
Cook the pasta (and peas)
In a large pot of boiling salted water, cook the 16 ounces small pasta shells according to the package instructions until al dente.
Add the frozen 1 cup peas to the boiling pasta water during the last 3 minutes of cooking.
Drain the pasta and peas in a colander and rinse under cold water to stop the cooking process. Set aside to cool.
Assemble the salad
In a large mixing bowl, combine the cooled pasta and peas, red onion, cherry tomatoes, and celery.
Flake the tuna into the bowl with the pasta mixture and add the chopped boiled eggs.
Pour the dressing over the salad and toss gently until everything is well coated.
Stir in the chopped parsley, if using.
Chill
Cover the bowl with plastic wrap and refrigerate the salad for at least 1 hour to allow the flavors to meld together.
Serve
Once chilled, give the salad a gentle stir. Taste and adjust the seasoning if necessary.
Serve the salad cold, garnished with ¼ cup fresh parsley if desired