Heat a 12-inch nonstick skillet over high heat and, when hot, add the olive oil. When the oil is hot, add the butter and, when melted and beginning to bubble, add the peppers and carrots and cook, stirring occasionally, until beginning to soften, about 3 minutes. Add the onion and cook, stirring, until crisp-tender, about 3 minutes longer. Add the zucchini and season with salt and pepper and cook until crisp-tender, 2 to 3 minutes longer. Serve immediately.