Combine the mayonnaise, lemon juice, dried dill, salt, and black pepper in a small bowl. Stir until smooth and set aside.

Finely dice the red onion and measure out the frozen peas. Set both aside.

Bring a large pot of salted water to a boil over high heat. Add the mini shell pasta and cook until almost fully tender, about 5 to 6 minutes. Add the frozen peas during the last 1 to 2 minutes of cooking. Total cook time is about 8 minutes.

Drain the pasta and peas well in a colander and allow them to cool. For faster cooling, spread them out on a sheet pan.

Transfer the cooled pasta and peas to a large bowl. Add the drained tuna, diced red onion, and prepared dressing. Fold gently until everything is evenly combined and coated. Serve immediately or refrigerate until ready to eat.