Stir together the salt and water in a non-reactive bowl until the salt has dissolved. Add onions, cover, and let stand at room temperature for 24 hours.
Stir together the brown sugar, flour, curry powder, turmeric, mustard powder, ginger, and allspice in a saucepan until evenly combined. Slowly stir in the malt vinegar, and bring to a boil over medium-high heat, stirring constantly until the mixture thickens, about 2 minutes. Once thick, set aside and allow to cool.
Drain the onions, and pack into two sterilized quart canning jars. Pour the warm sauce over the onions to cover. Seal with lids and rings. Leave in a cool, dark place 4 to 6 weeks to allow the flavors to develop.