Heat olive oil and butter in large shallow pot. Add the onions and thyme and toss with the oil. Place the lid on top and cook over medium-low heat from about 10 minutes to sweat the onions. Remove the lid and continue to cook over, stirring occasionally, 25-30 minutes until the onions are caramelized and golden brown. If the onions are cooking too fast, lower the heat. Add the vinegar, salt, and pepper and cook for 2 more minutes, scraping the brown bits from the pan. Season to taste