Combine cooking oil with flour.

In a frying pan, or saucepan, cook the flour and fat over medium heat stirring frequently with a heatproof spatula or a spoon.

When the roux reaches a rich brown color and smells nutty you can begin to slowly add the stock, about a cup at a time whisking constantly to avoid lumps.

Once you have all the stock incorporated it should be quite loose. Cook the mixture at medium low until it reduces by approximately on third.

If you want a thicker sauce use more roux. I would not cook it down further as the boxed stocks are very high in salt and the concentrated mixture can be overwhelmingly salty.

If not using the sauce immediately cover with a disc of cling wrap or parchment paper to prevent a scum from forming on top of the sauce.