Trim and discard the root from the leek. Cut the tough green tops off and discard.
Cut the white portion in half lengthwise, roughly chop, and rinse in calendar. Set aside.
Husk the corn. Cut the kernels from the cob, set aside.
Dice squash
Remove mushroom stem, cut cap in half and slice.
Finely mince garlic.
Pick the basil leaves and cut into thin strips. Discard the stems.
Zest and juice half the lemon.
Grate cheese with microplane.
In a pot over medium heat, warm 2 tablespoons oil or ghee until hot but not smoking.
Add the leeks, sprinkle with a couple of pinches of sea salt and cook, stirring occasionally, until tender, 5 to 7 minutes.
Add the garlic and cook until fragrant, about 30 seconds.
Stir in the sliced mushrooms and the Arborio rice and cook until the rice is opaque, 2 to 3 minutes.
Stir in ¼ cup wine or water, and cook until the liquid has been completely absorbed, about 1 minute.
Add broth, bring to a simmer. Place lid on pan and cook for 15 minutes or until most of the liquid is absorbed.
Add corn and squash cook for 3 minutes.
Add lemon juice and zest, parmesan cheese, basil and nuts. Cover and cook for 2-3 more minutes. Stir in walnuts and serve.