Cook the pearl couscous as per the directions on the package. I like to use chicken or vegetable broth instead of water for extra flavor.
While the couscous is cooking, mix the pesto and the juice of half of a lemon in a small bowl; set it aside.
When the couscous is ready, transfer it to a serving bowl and drizzle the lemony pesto over it. With two large spoons, toss it to evenly coat.
Add the chopped sun-dried tomatoes, onions, feta, and tuna to the couscous. Drizzle lemon juice from the other half of the lemon over it all and toss it again.
Crack some black pepper over the salad, then garnish with chopped fresh parsley.