Vegemite. Or Marmite if you are weird.

Chicken.

Distilled sterile water. There was nothing living in it. There was no living things before to add any flavours.

My guess is very, very bad. It's basically a collection of random oily organic compounds with all manner of other elements mixed in. So my guess would be something like used, burnt motor oil.

This implies that minerals and organic compounds have no taste, which is not the case.

Why would anyone downvote this question? It's a good one.

Or an everything bagel

Mmm everything oil.

Seawater, probably.

There were very few minerals, no complex organic compounds and very few simple organic compounds in the primordial soup. I severely doubt they'd give it any taste.

Potatoes.

Like nothing, as there was nothing to taste it yet.

A bit like pre-LUCA-um

Not true! Flavours existed, taste buds didn't yet.

probably poisonous, as there would be either compounds mixed into it.

I think there would be plenty of complex organic molecules. I mean you basically have a planet wide Urey-Miller experiment running for millennia with no life to eat any of the molecules generated, so they'd just... build up. I can't remember where I read it (so you'll just need to take my word for it), but some scientist posited that the pre-biotic Earth's oceans may have had a foot or so of oily sludge floating on them.

But it's important to consider that, even though they aren't eaten, these generated compounds aren't just idly siting around on this pre-biotic Earth. Waves are whipping the ocean sludge into an emulsion on every shore line of the world, tide pools are filling, getting baked by the sun and then washed into the ocean again. Lightning strikes are electrolizing the sludge and lava from primordial volcanos is scorching it. Most importantly with no ozone layer the sun's UV is blasting the Earth, breaking apart molecules or crosslinking them together into new more complex molecules.

Probubly similar to sewage

Everything bagels (fo)Я lyfe!

Likely warm mineral water with a strong saline taste, a slight metallic bitterness, and possibly a sulfurous aftertaste. So nasty.

I quite enjoyed this blurb here, fascinating stuff. I wonder if there are any documentaries about this or like Spore's Level 1 type stuff