Place the diced zucchini in a colander, toss with 1/2 teaspoon of the salt, and let drain for 10 minutes to remove excess moisture. Pat dry thoroughly with a clean kitchen towel.

Grill the corn on the cob over medium-high heat, turning occasionally, until lightly charred in spots, about 8 minutes. Let cool slightly, then slice the kernels off the cobs. If using frozen corn, thaw completely, pat dry, and char in a dry skillet over high heat for 2–3 minutes.

In a large bowl, whisk together the olive oil, lemon juice, white wine vinegar, Dijon mustard, remaining 1/4 teaspoon salt, and black pepper until emulsified.

Add the drained zucchini, grilled corn, red onion, and basil to the bowl with the dressing. Toss gently to coat. Fold in the crumbled feta until just combined – avoid overmixing so the feta stays in nice clumps. Taste and adjust with more salt, pepper, or lemon juice if needed; the salad should taste bright and well-seasoned.

Let the salad sit at room temperature for 5 to 10 minutes for the flavors to meld. Serve immediately, or refrigerate for up to 2 hours. Note that the zucchini will soften as it sits, so it’s best not to make this more than a few hours ahead.