Heat the oil in a medium-size skillet over medium heat. Add the onion and garlic, cover and cook until softened, about five minutes.
Transfer the vegetables to a 4-quart slow cooker, add the stock, tomatoes, tomato paste, sugar and bay leaves, and season with salt and pepper. Cover and cook on low for 5-8 hours. Adjust the seasonings and remove the bay leaves.
To serve, spoon about 1/4 cup of cooked couscous into the bottom of each bowl, ladle the hot soup on top and serve sprinkled with the basil.