Bring vegetable or chicken stock to a boil in a medium stock pot, then reduce heat to a low simmer.
Melt butter in a large, heavy bottomed stock 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. Gradually ladle in simmering vegetable stock, stirring continuously. Risotto will become "creamy" and slightly sticky, yet still firm in the center, or al dente.
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.