For truly enormous onions, choose Kelsae sweet onion seeds. This HEIRLOOM cross of two exhibition onions was developed in Scotland in the 1950s. Kelsae is a sweet, mild, white onion of the long day variety, suited to growing in northern regions. In 1995 one grower managed the world record of the largest onion ever grown at 16 lbs, 8 1/2 oz. Most growers will not achieve this size, but the bulbs will certainly be large. Kelsae keeps well into the following year. Although it was the result of a crossed parentage, Kelsae is not a traditional hybrid. It will produce viable seeds for seed saving.
Plant description from West Coast Seeds
Sow Ailsa Craig sweet onion seeds for huge, straw-yellow globes up to 20cm (8") in diameter! Firm, with mild, sweet flesh, Ailsa has fairly good short-term storage potential, certainly better than Walla Walla. Ailsa Craig Spanish onions are fine textured, and excellent for salads and sandwiches. It is tolerant of cold weather, so it can be ready for harvest before some other Spanish storage onions. Ailsa Craig was not a person, but rather a place. It is a stony island in the Firth of Clyde in Scotland. While the onion is not from there, it would certainly grow there, as it is adapted for growing between the 38th and 60th parallels, and that makes it an even better onion for northern growers.
Plant description from West Coast seeds
This is the classic large overwintering onion of the Pacific Northwest. A very large, juicy onion with a mild sweet flavour. It can be planted in the spring if given a really early start indoors, or in August to overwinter. Spring planted Walla Walla sweet onion seeds result in bulbs that are not quite as mild as the overwintered ones. And spring sown Walla Walla onions may not size up as well. We recommend that home gardeners should wait and plant for overwintering onions. This variety is hardy to -23°C (-10°F). It is most definitely a long day onion, suitable for planting between the 35th and 55th parallels. It might not have good storage ability, but man, it makes beautiful caramelized onions!
Seeds description from West Coast Seeds