Bring a large saucepan of salted water to a boil. Cook pasta according to the package directions. Add in the peas for the last couple of minutes of cooking time, drain, and return to the pan to evaporate the remaining liquid. While the pasta is still hot, squeeze the juice of ½ a lemon over the pasta and peas and stir well. Set aside to cool.
Meanwhile in a large bowl, combine the remaining ingredients until mixed well. Add the pasta and peas to the tuna mixture and gently combine (so you don't crush the peas). Taste and add salt and pepper to season. Refrigerate before serving.
Tip: This makes a fantastic make-ahead lunch or light supper!