Leaves are yellowing and drying out, too late or saveable?

keep it well watered...consistently moist but not wet. It's hard to tell how beet-friendly that soil is. Beets prefer sandy loam to clay and gravel. What is the soil?

Maybe not ideal, wouldn't say it is sandy or clayish but is a bit gravelly. I'll try to maintain the moisture. It isn't a bug that is causing the issue is it?

no

Thank you

It's leafminer damage, and pretty serious to boot. I suspect you are dealing with Spinach leafminers which, in spite of the name, love feeding on chard/beet/etc.

Those opaque areas you see are the "mines" where the larvae feed: it's dead tissue that will later turn dry. Use a spinosad based insecticide since it's effective but has minimal/no toxicity on bees, puppies and leprechauns.

El Pollo clearly looked more closely than I did - and he likely won't like my further response:
What I should have said was whether insect or not, I would not use an insecticide as, in my beet-growing experience beets always grow out of whatever troubles them. That said, they can't compensate for poor soil. They are a root crop after all and want loose loamy fertile ground.

The beetroot has recovered but what are these on my brassicas and how do I deal with them?

Oh, more evidence of sex in the garden.

Those are eggs; and not the kind to scramble with bacon. Likely cabbage white butterfly. Eliminate them tout suite.

You can simply 'smudge' them into non existence...it doesn't take any particular skill or chemicals. Check the undersides of your other brassica plants.

Thank you