Heat a large pot over medium-low heat and add the olive oil and butter. Once melted, add in the onions, garlic, salt and pepper, stirring to coat. Cook for 5 to 6 minutes, until the onions softened and begin to turn translucent. Increase the heat to medium and add in the flour, whisking constantly for 1 to 2 minutes until it is evenly dispersed to create a roux. Cook the roux until it is fragrant and begins to turn golden in color.