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Finely chop the mushrooms using a food processor. Pulse rather than puree.
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Heat the oil in a wide skillet and add the mushrooms.
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Cook over medium-high heat for 12–15 minutes, stirring, until much of their moisture has evaporated and they begin to brown.
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Add the garlic and cook another minute.
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Add vinegar, molasses, mustard, spices, salt, and 1/4 cup water.
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Reduce the heat and simmer uncovered for 15–25 minutes until the mixture is thick and concentrated.
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Blend until smooth, adding a little water if necessary.
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Return to the pan briefly if it needs more reduction, then taste for acidity, sweetness, and salt.
