Heat the olive oil in a saute pan set over medium-high heat. Once hot, add the onion, salt and pepper and cook until brown and fragrant, about 3 minutes. Throw in the dry couscous and continue cooking for another 2 to 3 minutes, stirring often, until the couscous is slightly toasted.
Add the chicken broth, lemon zest and lemon juice and bring to the boil; Reduce the heat to medium-low, cover and simmer for 8 to 10 minutes – until the couscous is al dente and about 3/4 of the broth has been absorbed.
Remove from heat, stir in the cooked chicken, Parmesan cheese, green peas and arugula and mix until well combined. Cover and allow to rest for about 5 minutes, until the arugula is completely wilted and the peas and chicken are heated all the way through.
Serve immediately, garnished with additional grated Parmesan cheese if desired.