Bring a large pot of generously salted water to a boil. Cook rotini pasta according to package directions until al dente. Drain and rinse briefly under cool water. Let cool for 5 minutes.
In a large mixing bowl, whisk together mayonnaise, Dijon mustard, lemon juice, salt, and black pepper until smooth.
Add the slightly warm pasta to the bowl and toss to coat evenly with the dressing.
Fold in the drained tuna (broken into chunks), diced celery, red onion, thawed sweet peas, and chopped hard-boiled eggs. Stir gently until all ingredients are combined.
Cover the bowl and refrigerate for at least 30 minutes to allow flavors to meld.
Before serving, stir the salad and taste for seasoning. Adjust salt, pepper, or lemon juice as needed. Garnish with fresh dill or parsley if desired.
