Heat the oil and butter in a medium saucepan over medium-high heat. Add the mushrooms and cook, stirring occasionally, until softened, about 7 minutes. Add the onion and cook 5 minutes more.
Add the chicken broth and the salt. Bring to a boil and stir in the couscous. Immediately remove the pan from the heat, cover it, and let it stand until all the liquid is absorbed, about 5 to 10 minutes.
Uncover and fluff the couscous with a fork. Transfer to a serving bowl, garnish with the basil, if desired, and serve immediately.