Make the dressing by combining mayonnaise, lemon juice, dill, salt, and pepper in a bowl.

Finely dice the red onion and measure out the frozen peas.

Bring a large pot of salted water to a boil over high heat.

Add the pasta and boil until almost fully tender (about 5-6 minutes), then add the frozen peas.

Cook the pasta and peas together for the last minute or two until fully tender.

Drain the pasta and peas well in a colander and let them cool until no longer steaming.

Combine the cooled pasta and peas with diced red onion and drained tuna, then pour the dressing over and toss gently.