Preheat oven to 375 degrees F. Spray 9x13" pan with cooking spray.
Combine couscous, boiling stock, and salt in a large bowl and let stand for 5 minutes. Fluff with a fork.
Saute onion and garlic in olive oil until the onion is translucent. Add the drained diced tomatoes and cook for about 10 minutes more.
Stir the tomato/onion/garlic mixture into the couscous. Add reserved tomato juice, basil, raw spinach, and pepper. Mix until well combined.
Spread half the couscous mixture in the 9 x 13 pan. Sprinkle on the light mozzarella cheese, then top with the remaining half of the couscous. Cover the dish with foil, and bake at 375 degrees for about 30 minutes, until hot and bubbly.
If you like mushrooms, you could add 1/2 a pound or so of sliced fresh mushrooms to the onions and garlic as they saute in the olive oil.