Preheat the oven to 400 degrees F. Line a sheet pan with parchment paper.
In a large bowl, stir the tomatoes, harissa paste, 1 tablespoon olive oil, 1/2 teaspoon kosher salt and 1/4 teaspoon black pepper.
Arrange the tomatoes in a single layer cut side up on the prepared sheet pan.
Roast the tomatoes for 20-25 minutes until they are wrinkled, but still juicy in the middle.
While the tomatoes are in the oven, make the dressing, In a large bowl, 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 couscous into the lemon tahini dressing.
Fold the tomatoes, cucumbers, scallions and parsley into the couscous.