Heat the oil in a heavy-bottomed Dutch oven over medium-high heat and, when hot, add the flour. Whisk to combine and continue to cook, stirring constantly, until a medium roux is formed (it should look a bit darker than peanut butter), about 10 minutes. (If the roux begins to brown too quickly, reduce the heat to medium or medium-low and take your time - it is important that the roux not be burned at all of the stew will have a bitter taste.) As soon as the roux is the right color, add the chopped onion and cook until soft, stirring occasionally, 4 to 6 minutes. Add the garlic and cook for 2 minutes. Stir in the stock, little by little, and bring the sauce to a gentle boil. Add the bay leaves, black pepper, cayenne, thyme, and 4 teaspoons of the salt and reduce the heat so that the sauce just simmers. Cook, stirring occasionally, until the floury taste is gone, 30 to 45 minutes.