Beetroot

I always do beetroot from modules.  My soil is clay like yours and I find seeds often rot before they get going, if sown direct in the ground.  I use the size where you get 12 modules to a seed tray and sow 3 seeds in each.

Plant them out when they get going and grow together as a clump with about a hand width of space between clumps.  Beetroot are veg that grow as swollen stems rather than true root veg, so it is not so critical if you leave them in modules to grow a couple of decent sized leaves each.  They won't fork or grow as a tangled mess like carrots or parsnips would.

You get smaller beetroot but plenty of them and they don't take up much room on the plot 

where in modules       lol