1. In a bowl, dissolve 1/4 cup salt in 1 quart water. Add the onions. Weight them gently with a plate that fits inside the bowl. Let them stand 8 to 12 hours.
2. Drain the onions, and peel them. Return them to the bowl. Make a brine with the remaining salt and water, pour it over the onions, and weight them gently again. Let them stand 2 days.
3. In a nonreactive saucepan, bring the sugar and vinegar to a boil. Let the liquid cool.
4. Drain and rinse the onions, and drain them well again. In a 1-quart jar, layer the onions, peppercorns, allspice, pepper flakes, and bay leaf. Cover them with the cooled, sweetened vinegar. Cover the jar with a nonreactive cap, preferably all plastic.
5. Refrigerate the jar for at least 1 month before eating the onions. They will keep for at least 6 months.