Mix zucchini, onion, and 1 tablespoons salt in a medium bowl
Let sit 10 minutes, then wring out as much liquid as possible from zucchini and onion in a kitchen towel
Mix with leek, egg, panko, cilantro, and thyme in a clean medium bowl; season with pepper
Pour oil into a large skillet, preferably cast iron, to a depth of 1/2" and heat over medium heat until oil bubbles when a pinch of zucchini mixture is added
Working in batches, drop spoonfuls of zucchini mixture into skillet and gently flatten with a spatula into 2" patties