You probably know that bolting is a common problem with onions and is usually caused by unsettled weather conditions, a cold spring followed by hot weather seems to make it worse. Loose soil can also cause bolting. The plant roots are disturbed and, thinking it is starving, the onion reacts by trying to spread its seed. Once started the only thing to do is to cut the stalk off an inch or two above the bulb. Use these onions first as they will not store well. A thought for next year's overwintering onions - bolting can be suppressed by top dressing with a nitrogen rich fertiliser in January.
I haven't seen this happen before in onions but I've read on the gardening forums that it does happen! It's the amount of daylight that makes an onion trigger into developing the bulb so I'm wondering if it's a cold spell of weather that have caused them to divide.
There is another kind of onion called the potato onion (also known as multiplier onion) this is a variety very similar to the shallot, although producing larger bulbs. It was very popular in the past, but like many of the old varieties they have died out for other disease resistant varieties. Your onions sound very much like these. I'm just wondering if you have been supplied with shallots instead of your Red Baron onions?