mrhoe: What about Ailsa Craig (named for the small, rock island of the coast of England), a mild, sweet, long-keeper that grows to a kilo in 100 days? (Just a thought.) I don't like the sound of "medium to heavy clay". The best soil for onions is a sandy soil, where the bulbs can easily increase in size (with little resistance from heavy soil). Is there a possibility of incorporating sand into your soil, to lighten and loosen it? That would be the perfect scenario. Also, planting as EARLY as possible, is the next consideration. I don't know about your particular area, but I try and get my onions plants in the ground sometime in early March, if possible. I'm in an area where our last frost date is about May 1--but onions can tolerate quite a bit of cold--many varieties will Winter over in my zone. To get them in the soil in March, here, requires raised beds (which I have). In attempting to grow large onions, it would be superb if you could do the same. Also, compost incorporated into the soil (along with sand--and preferably accomplished the preceding Fall), makes for a great onion bed. Onions also prefer a high phosphorous fertilizer--growers here recommend 10-20-10, although many organic (biological) growers use a compost with well-rotted manure in the place of chemical fertilizers. Growing large onions requires time--a 100-days-to-maturity variety planted in early March has the months of March through June to attain its size. Onions are finished growing here after the Summer Solstice, so one has only 'til June 21 to squeeze in those 100 days. So, raised beds with plenty of sand and organic matter, which lightens the soil and allows it to dry out much earlier, will allow you to plant early and grow large onions. Wait 'til mid-May and you'll grow onion sets, instead of onions.

Cheers marylandmojo
I will try what you have said about the raised beds sounds as that is the best way to go with my type of "soil".

Of course, folks here grow onions in any type soil--but they reach their size potential when they can expand freely in loose soil. I grow a "sweet" onion here nicknamed Vidalia (its true name is Granex)--named after Vidalia County, Georgia, one of the southernmost counties in Georgia, near the Florida line, where it's mainly grown. It grows there in almost pure sand; that same sand (soil) permeates the southeastern United States, particularly the (Atlantic) coastal states of South Carolina, Georgia, and Florida. They Fall-plant Vidalia onion seed, there, right about this time of year, and their onions will reach 1/2 kilo in size by harvest time next May or June. (It's not a giant onion; it's grown mostly for the sweet, very mild, taste.) I plant Vidalia onion plants or sets (starts) here, in the mid-Atlantic region as near the first of March as I can, and they'll mature at an average weight of 1/4 kilo, at the Summer Soltice. So, their milder growing season (in Georgia) is twice the length of mine, and their (Vidalia) onions achieve twice the size. It would be interesting to compare the size your onions achieve in your present soil and in soil (later) amended with sand. Can you get your onions in the ground around the first of March, in your area, or would that require raised beds? It certainly requires them here, as the soil is usually too wet on that date (and sometimes its covered with snow).

When did Ailsa Craig move to England it is a volcanic plug island 3 miles from the Ayrshire coast in Scotland and is also 4 miles form the town of Troon in Ayrshire,my family owned the island for many years and we quarried the granite for production of curling stones,I still own the island and cant believe that it was moved 80 miles south without me noticing as my house is still on it and thats where this post is being sent from....