This recipe for mashed zucchini with chiles and Cheddar embraces zucchini’s tendency to become very soft when cooked.
Heat oil in a large nonstick skillet over medium heat. Add zucchini and onion; stir to coat. Cover and cook, stirring occasionally, until very soft and lightly browned, 12 to 15 minutes. Stir in chiles and salt; cook until heated through, 1 minute more.
Transfer to a medium bowl. Mash with a potato masher until chunky, not completely smooth. Stir in cheese and serve immediately.