Red cabbage

What you could do is sow a fair few seeds in a largish pot and weed them down to to best looking 15 to 20 plants as they grow. then plant it out somewhere and eventually collect the seeds from them. They won't be able to grow massive but their seeds should be the same as if they had i would think, particularly if you thin them out after the seeds have set.

They can still grow quite large like this though as I once had about 5 PSB in a 4inch pot and had left it till the roots were all entwined. So I planted it as it was and they grew to about normal size but flopped over in different directions taking up quite a bit of room - but less than if i had planted them out individually.