In a large skillet, heat the butter over medium heat. Once it's melted, add the onion and mushrooms. Cook for 3 or so minutes, then add the zucchini and corn. Cook until everything has a chance to soften, about 5 or so minutes, then stir in the garlic and cook for another minute or so before removing the pan from the heat. Make sure to strain out any excess liquid in the pan, then place the veggies in a large bowl and let them cool for just a few minutes.