Bring stock to a boil in a saucepan, then reduce heat to a low simmer.

Melt butter in a large, heavy-bottomed stock pot over medium heat. Add onion and cook until softened, about 2 minutes.

Add rice and cook until lightly toasted, about 2 minutes, stirring constantly.

Gradually add simmering stock, stirring continuously. Risotto will become creamy and slightly sticky. When almost finished, stir in zucchini, sun-dried tomatoes, and thyme, adding stock as needed and stirring continuously. The rice should be tender yet firm to the bite.