I planted some beetroot from store bought seedlings in November, and they're fine. But this chap grew up with them, and I don't remember if it came with the others, or just grew from where ever.
I just don't know what it is. Doesn't look like a turnip to me. And the leaves looked like a kind of spinach which is why I left it so long in the ground occupying valuable real estate.
And a single leaf against a dibber with 2.5 cm markings