In a large heavy pot on medium-high, heat two tablespoons of olive oil. Add the couscous and saute in the olive oil until it turns a light golden brown (3-4 minutes). Add 3 ½ cups of boiling water and a large pinch of salt and stir it around. Cook according to package directions until al dente (usually about 8 minutes). It will likely soak up all the water, but if there is any water left, drain it well in a colander and set aside in a large mixing bowl to cool.
When the couscous has cooled, fluff it with a fork and add the olives and scallions. Add ¼ cup of pesto and toss to coat. If it looks too dry, add more pesto until it’s nicely coated but not laden. Taste for seasoning. Scatter the mozzarella and tomatoes over the top, and garnish with basil leaves before serving.