The thinly sliced vegetables lend to the side dish's overall lightness.
Heat oil in a large skillet over medium-high heat. Add onions, celery, and salt. Cover; cook, stirring occasionally, until onion is translucent, about 5 minutes. Add 1/2 cup water; cover, and cook until celery is tender, about 5 minutes.
Stir in zucchini. Season with pepper. Cook, stirring once or twice, until zucchini is very tender, 5 to 6 minutes. Using a slotted spoon, transfer vegetables to a serving platter, and drizzle with oil. Serve warm or at room temperature.
Use a mandoline or a very sharp knife to cut the celery and zucchini.