Bring 1/3 cup water to a boil in a hot pot. Stir in couscous and a pinch of salt, cover and turn power off.
Let it sit undisturbed until water has been absorbed, at least 5 minutes. Fluff with a fork.
While couscous is steaming, mix pesto, lemon juice and lemon zest, if desired, in a wide, shallow bowl. Season with pepper. Toss in tomatoes and cucumbers.
Add couscous to bowl along with pecans, if desired, and toss again. Season with salt and pepper.