Cook the Pasta and Egg: Bring a large pot of salted water to a boil. Add the pasta and the egg. Cook the pasta until al dente, following package instructions. Drain the pasta and egg, then rinse the pasta with cold water or add ice cubes to stop the cooking process. Peel and chop the egg.

Prepare the Vegetables: While the pasta cooks, finely dice the onion, shred the carrot, and chop the cilantro.

Mix the Tuna: In a small bowl, break up the tuna with a fork and mix it with the mayonnaise.

Combine the Ingredients: In a large bowl, combine the cooled pasta, chopped egg, shredded carrot, diced onion, tuna-mayo mixture, and lemon juice. Season with salt and black pepper to taste. Toss until everything is well coated.

Chill and Serve: Cover the bowl and refrigerate for at least 1 hour. Serve cold, garnished with additional cilantro if desired.