Pile zucchini and onion in the middle a piece of cheesecloth or several paper towels. Pick it up and squeeze as much moisture out as you can. Transfer to a bowl and use a fork to stir in breadcrumbs, egg, and salt until well combined. If mixture is too wet to hold together, add a few more breadcrumbs.
Preheat a heavy skillet over medium heat for a couple of minutes. Add oil and drop batter by about 2 tablespoonfuls at a time to make fritters. Don't overcrowd the pan, leave enough space between them to flip easily. After about 3 minutes, when the undersides are golden, flip and continue cooking on the other side until it's golden, too, and centre feels firm when carefully pressed (about 3 minutes more). Repeat with more oil if needed until all fritters are cooked. If at any time they're browning too quickly, simply reduce heat.
Keep warm in a 200º oven if desired, or serve immediately. Leftovers can be frozen and reheated in the oven.