Blanch the arugula in a large pot of boiling water for 10 seconds. Dunk the basil in the pot too, then use tongs to immediately transfer the arugula-basil mixture to a colander. Rinse under cold running water to stop the cooking, until no longer warm to the touch. Set aside to drain.
Return the pot of water to a boil. Add the couscous and cook, stirring occasionally, until al dente, 8-10 minutes. Drain, then spread out the couscous on a rimmed baking sheet and drizzle with a a bit of olive oil, and toss to coat. Let cool to room temperature.
In a small skillet over medium heat, toast the pine nuts until golden brown, about 2 minutes. Remove from the pan and set aside to cool.
Squeeze remaining water from the arugula-basil mixture. Transfer to the bowl of a food processor. Add the pine nuts, garlic, lemon zest, lemon juice, salt, pepper and olive oil. Pulse until smooth, adding a little water a bit at a time if the consistency is too thick. Taste, and adjust with more salt and pepper if needed.
Transfer the couscous to a large serving bowl. Add some of the pesto (you may have some leftover) and stir to coat. Fold in the tomatoes, cucumber and onion. Serve.