Combine all marinade ingredients in a large pot and bring to a boil; let boil for 15 minutes. Strain liquid, discarding solids, into a bowl. Let marinade cool to room temperature; chill in refrigerator or use an ice bath until marinade is cool enough to add to the raw prawns.

Trim prawns, using scissors. Place thinly sliced onion and jalapeño slices in a single layer in a large container with lid. Add prawns in a single layer over onion; pour chilled marinade over prawns. Cover and place in refrigerator for 24 hours.

Remove container from refrigerator. Strain marinade through a fine mesh sieve and into a pot. Place prawns back in refrigerator. Heat marinade and let boil for 5 minutes. Chill completely and pour marinade over prawns; place container back in refrigerator. Repeat every 24 hours for two more days.