In a saucepan, bring 2 cups water and the lemon juice to a boil. Stir in the couscous, then cover and let stand for 15 minutes. Toss with the grapes; season with salt and pepper.
Bring a pot of salted water to a boil. Add the broccoli and cook for 3 minutes; drain. Toss with 1 tablespoon olive oil, salt and pepper.
In a large skillet, heat the remaining 1 tablespoon olive oil over medium-high heat. Season the tuna with salt and pepper and cook, turning once, until browned, about 8 minutes; transfer to a plate.
Add the chicken broth to the skillet and cook, scraping up any browned bits, until thickened, about 3 minutes. Whisk in the mustard, season with salt and pepper and cook for 2 minutes more; stir in the scallions. Spoon over the tuna and serve with the broccoli and couscous.