Slice your zucchini, and place in a mesh strainer and toss with salt. Set aside for 10 minutes to draw the moisture out. After 10 minutes, pat the zucchini dry.
Heat a grill or grill pan over medium-high heat. Brush the zucchini, corn, and red onions with the oil and season with salt and pepper.
Arrange the veggies on the grill and cook, turning occasionally, until the zucchini turns golden brown, about 3 to 5 minutes, until the corn kernels are browned in spots, 6 to 8 minutes, and until the red onions are charred all over and tender, 9 to 10 minutes. Transfer vegetables to a cutting board and let cool slightly.
While the veggies are cooling, add the lemon zest, lemon juice, agave or maple syrup, if using, mustard, olive oil, garlic, salt, and pepper to a mixing bowl and whisk until combined.
If you are using the kale, slice it, place it in a large mixing bowl, and massage it for 5 minutes. Add in your other greens to the mixing bowl.
After the charred onions have cooled, slice them into thin strips, cut the corn off the cob, and add them to the mixing bowl along with the zucchini, feta, and avocado. Add the dressing the toss the salad to combine.
This is best fresh. If you want to have this multiple nights, only dress what you plan on eating. Store the leftovers separately in airtight containers in the fridge for 3 days.