Preheat the oven to 400 degrees F. On a baking sheet, toss the zucchini, yellow squash and peppers with 1 tablespoon olive oil, 1/2 teaspoon kosher salt and 1/4 teaspoon black pepper. Spread the vegetables into a single layer across the pan. Roast for 25-30 minutes until they are browned at the edges.
For the dressing, whisk together the lemon juice, 1/4 cup olive oil, tahini, garlic, 1/2 teaspoon kosher salt and 1/4 teaspoon black pepper.
For the couscous, heat 1 tablespoon olive oil in a large saucepan over medium heat. Toast the couscous stirring until it golden, about 2-3 minutes. Add the water and bring to a boil. Reduce heat to low, cover and simmer until the couscous is tender and most of the water has been absorbed, about 10 minutes. Drain any remaining water and transfer the couscous to a large bowl.
Stir the lemon tahini dressing into the couscous.
Then add the roasted vegetables, scallions and parsley to the couscous, stirring to combine.