Combine cooking oil and flour in a deep, microwave-proof dish and mix well.

Microwave on high, stirring every 3 minutes, until dark brown (chocolate color). Be careful, as it gets very hot.

Set aside to cool the roux.

Peel and cut potatoes into medium-sized chunks.

Heat water to boiling in a large Dutch oven.

Gradually add the roux to the boiling water, stirring well until smooth and medium consistency.

Add corned beef, potatoes, onions, salt, and pepper.

Bring the mixture to a slow boil.

Reduce heat to simmer, cover, and cook until potatoes are done, about 30 minutes.

Add more water if too thick or more roux if too thin.