Well, a Carya of some sort. Either pecan or hickory. I'd lean towards pecan, but some hickories are similar.

Where is it growing?

tj

Thanks TJ -- the tree is growing in Sydney Australia

20 mm long sounds very small for a Pecan. Basically an inch. What's inside the pod?

Yes, it does seem small but I couldn't see any larger specimens either on the tree or on the ground. Perhaps a Carya / Hickory hybrid? - anyway, this is what the seed looks like inside. The pod is likely very immature and the brown material you can see is very soft yet it looks like the outer green layer would be removable at some point if it were to dry out and harden.

Yes, that does look like an immature nut. That accounts for the misleading size. I thought it was fully grown.