Bring vegetable or chicken stock to a boil in a medium pot, then reduce heat to a low simmer.
Melt butter in a large, heavy bottomed pot over medium heat. Stir in onions and cook for 2 minutes, or until softened. Add the rice and cook for another 2 minutes, stirring constantly, until lightly toasted and covered in the oil. Gradually ladle in simmering vegetable stock, stirring continuously. Risotto will become "creamy" and slightly sticky, yet still firm in the centre.
When almost finished, stir in the zucchini, sun-dried tomatoes, and thyme, adding stock as needed and stirring continuously. Stir in basil and 3 tablespoons cheese just before serving. Divide risotto among 6 bowls, sprinkle with remaining cheese, and season with pepper to taste.