Heat the oil and butter in a large skillet over medium heat. Add the onions and cook, stirring occasionally, until just softened, about 5 minutes. Add the garlic and cook, stirring occasionally, for 1 minute. Add the zucchini, squash, Italian seasoning and a generous pinch of salt. Cook, stirring occasionally, until the zucchini and squash are tender and beginning to brown in some spots, about 10 minutes. Season with salt and pepper. Serve warm.