1. Whisk together olive oil, lemon juice, garlic and 1/2 tsp. salt in a small bowl.
2. Bring broth to a boil in a heavy pan. Stir in couscous, cover, reduce heat to low and simmer until tender, 12 to 15 minutes. Transfer couscous to a large cookie sheet, spread it out and let cool for 15 minutes.
3. Transfer couscous to a bowl and stir in broccoli, tuna, olives and parsley. Toss with dressing to coat. Season with additional salt, if desired. Serve immediately, or cover and refrigerate for up to 1 day.