For the herb salt: Combine the salt, oregano and thyme in a small bowl and mix to blend.
For the grilled zucchini: Brush the zucchini planks on both sides with olive oil and sprinkle on both sides with some of the herb salt. Transfer the zucchini to a grill pan set over medium heat and cook until tender, about 5 minutes, flipping halfway through. Remove the zucchini to a serving plate and sprinkle with the feta.
Save any leftover herb salt. It makes a great all-purpose seasoning.