It looks like a white beet.

Now that you mentioned that, I looked at other types of beets. I think it is a sugar beet that was mixed in with my red beets.

There are edible white or pale beets. Google Chioggia beets, which have white/red layers, and the coarser leaves you see in the picture. I grow both regular and Chioggia.

The thick mid ribs make me think this is not a beet, but what I would call spinach beet. This is a misnomer as it isn't spinach at all. It is really a kind of chard. It is the leaves which are eaten, not the root. Old chard plants can get massive roots, especially if overwintered. Sometimes they are really tough to pull out at the end of the season.

Here is a link that might be useful: Spinach beet

Looks like a chard, which is nothing more than a beet that has been selected for heavy stalks on its leaves. Or you might say a beet is generally a red chard that has been selected for its roots instead of its leaves and their leaf stalks.