Heat the oil in a 1-or 2-quart saucepan over medium heat. Add the onion and garlic, cover and cook over low heat for about 5 minutes to wilt the onions. Check halfway through to make sure they are not browning.
Raise the heat to medium again, stir in the flour, cumin and black pepper, and cook, stirring, for 2 minutes to cook the rawness out of the flour. The onions will tend to ball up into clumps, but that does not matter. When the onion-flour mixture just begins to color, remove pan from heat and gradually pour in the broth, stirring constantly to prevent lumps-a wire whisk helps here.
Add all the remaining ingredients. Return pan to heat and bring to the boiling point, then cover and simmer over low heat for 30 minutes, stirring occasionally. The finished sauce should be thick enough to nap a spoon-if too thick, dilute with a bit more broth or water.
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