In a large skillet, heat the vegetable oil over medium heat. Add the onions and cook, tossing and stirring occasionally, for 15 to 20 minutes, or until softened. Reduce the heat to low and continue cooking, stirring occasionally, for another 50 to 60 minutes, or until the onions are completely soft and caramelized. They will look dark brown and mushy. Remove from the heat and add the salt, pepper, and thyme and mix well to combine. The onions will keep in an airtight container in the fridge for up to 1 week.