A large asteroid is discovered that contains a wealth of minerals.
Hundreds, and later a few thousand, miners etc. set up to exploit these resources - initially living in prefab modules or hastily constructed caverns.
Most of this could be TL9 to 10 initially, although mining colonies built during the RoM may have begun with TL12 infrastrucure in place.
The miners become increadibly wealthy thanks to the value of the raw materials to local industry.
Thanks to high shipping costs it is cheaper to set up the refineries on the surface of the asteroid - cheap fusion power makes this a possibility.
As the minerals are removed, and larger caverns produced as a result, more and more space is available for living areas and artificial farms - making the colony self sufficient in all foodstuffs.
Many of these caverns have very high ceilings with articicial lighting producing a "natural" vista, and grav plates for normal gravity.
As more and more support services for the miners, refinery workers, and farmers are built up you get more and more families - as a result you have a market for finished goods, so manufacturing industry is established.
Over the centuries the population keeps growing. There is wealth, there, are raw materials, there is agriculture, and there is manufacturing - all in a self contained little world a few hundred km across.
Many of the occupants would never visit the surface even - no need.
Most wouldn't see the point in selling up and paying for passage to a "real" world where the standard of living may not be as high, and getting a job could be difficult.
How many such asteroids in a rich belt?
Trouble is you can only realistically use this scenario a few times before it's as stupid as "the Ancients! did it