In a medium saucepan, bring the chicken stock to a simmer over medium heat. Reduce the heat to low to keep warm.
In a large skillet, heat the olive oil. Add the leeks, shallot, garlic, and mushrooms, and cook over medium heat, stirring occasionally, until the leeks are softened, about 6 minutes. Add the couscous and cook, stirring, for 2 minutes. Add the wine, stirring until absorbed. Add 1 cup of the warm stock and cook, stirring, until nearly absorbed. Continue adding the stock 1/2 cup at a time, stirring until it is nearly absorbed between additions. Risottos are done when the couscous is al dente and suspended in a thick, creamy sauce, about 20 minute. (I ended up with a 1/2 cup stock left in the saucepan.) Stir in arugula until wilted. Stir in the walnuts, lemon zest, 1/2 the lemon juice, basil, scallions, and cheese. Season with salt and pepper. Serve immediately.