Looks like a Tomato or Potato mutation to me.

Bolting spinach?

Not with those flowers. They're Solanaceae.
Are the flowers definitely connected to the leaves? It couldn't be a tomato growing up through a silver beet or chard could it?

A tree-type (strong-stemmed upright, usually narrow) dwarf tomato. ("Dwarf" refers to plant height, not fruit size.) Many/most tomatoes of that type have rugose foliage. The picture shows leaves more rugose than any tomato leaves I've seen, but it's not impossible.

Do you know if there were tomatoes growing there last year? Failing that, someone may have discarded part of a tomato there, or there might have been tomato seeds that survived incomplete composting.

You could ask at the Tomato forum ... but keep in mind that it's not possible to identify particular varieties with certainty, and this one may have grown from crossed seed.

AFAIK, there are no potatoes with yellow blossoms.