I start mine in cells, 4 seeds to a cell, then you can bring them on under cover, and when they fill the cells just plant them out. Seen this on Gardeners World.

We do it this way too, just do not disturb the roots when planting the cells out.

Must admit that I never do this. Have found that they do very well when planted directly in situ being well spaced so no need for too much thinning. Best to wait untill the soil warms up then they grow away quickly without a check in the growth. My local garden centre sells beetroot, carrot and parsnip in cells but I would imagine that the tap root would be easily damaged when transplanting and a lot more expensive than a packet of seeds.

Definitely do not buy them from G.C's. They are always very poor specimens, having been left too long without water etc. in poor light and too much heat.
We only do it for the first seed sowing, the late Beetroot seeds go straight into the soil. We do it like that as we are in a very cold area and need the extra growing time this method gives us.