Add the oil and butter to a large Dutch oven or soup pot over medium-low heat. When the butter is melted add the onion and cook, stirring occasionally, unto soft but not browned, about 7-8 minutes.
Increase the heat to
medium-high and bring the soup to a simmer while stirring to make sure the flour is not sticking. Reduce the heat to low, cover and cook for 40 minutes.