In a large, heavy-bottomed saucepan or skillet over medium heat, heat the butter and oil. When the butter has melted, add the sliced leeks and cook, stirring often, until slightly softened, about 5 minutes. Stir in the zucchini and cook, stirring occasionally for about 5 more minutes. Add the corn, the lemon zest, and the thyme sprigs to the pan. Add the rice and salt, stirring often, until the rice grains are starting to turn translucent, about 2 minutes.
4 tbsp unsalted butter, 2 tbsp extra-virgin olive oil, 2 medium leeks, 3 zucchini, 1 cup corn, lemon zest, 4 thyme sprigs, 1½ cup arborio or Carnaroli rice, 1½ tsp fine sea salt
Pour in the wine, and cook until wine has been absorbed. Begin adding the broth, a scant cup at a time- just enough to cover the rice, and stir until the broth is nearly absorbed. Stir in more broth a bit at a time, adding more after the previous amount is nearly absorbed, repeating this until the rise is tender but still tastes chewy, about 20-25 minutes. You should be able to see the rice transform as you're stirring and adding more broth. The grains will plump and whiten, becoming almost creamy.
⅓ cup white wine, 6 cups vegetable broth, chicken broth, or water
When there's about a cup of broth left, taste a few of the grains, it should be firm and a little chalky at the center, add the cream, the gruyere, and the Parmesan and continue to stir until the cheeses are fully incorporated. At this time, add more of the broth to achieve a creamy consistency (you might not use all of it). Remove the stems remaining from the thyme sprigs at this time.
⅓ cup heavy cream, ¾ cup gruyere cheese, ⅓ cup freshly shredded Parmesan cheese, fresh ground pepper
Risotto will be creamy but shouldn't be runny. It will mound in a bowl and some of the creaminess will spread. If it seems too runny, continue heating through another minute or two.
Spoon the risotto into shallow bowls, adding extra Parmesan, and grating fresh black pepper before serving.