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Explanation: I've been in the food business internationally for quite some time and have encountered many situations such as this. Though the actual word may vary a bit from "Polvo de Ajo" to "Polvora de Ajo" to "Ajo en Polvo" the most generically used term is the above. I would recommend that you use the same for your other inquiry, "Polvora de Cebolla".
Que le vaya bien!