In message , Mel
writes
There will also be three main beds, running N-S, each 15 feet long x 5 feet
wide with a 2 foot pathway between each bed (totalling 19 feet wide).

Unless you have long arms, a 5 foot bed is probably too wide to reach
the middle from the paths (4 foot is the recommended size, our
experience would lead me to concur with this.

Question -- is it better to grow one 15 foot row of onions, and one 15 foot
row of leeks, and one 15 foot row of carrots, and one 15 foot row of
parsnips, rows running north to south the full length of the 15 foot bed?
Or, should I treat the bed as 3 separate blocks (each 5'x5') and grow three
5 foot rows of onions, and three 5 foot rows of carrots, etc.,

I think block growing works better on raised beds than rows.

Sorry to be so clueless, but this is my first allotment and first attempt at
growing veggies.

don't worry, you try something, if it doesn't work so well you try
something different next year.
--
Chris French