It's not avocado . Is very bittet

Unripe walnuts possibly?

The tree is big and quite distinctive. The fruits are hard, the size shown and look nothing like a walnut at the moment.

That would be my thought too Buddlejagare

Any pics of the foliage, Heidi?

Osage orange tree.

Quince maybe?

If it's a walnut, then your hands will stain where you have touched the cut sides.

Ooh will take more photos when wevisit again today! It's a huge tree!

I'd like to see a picture of an intact fruit, so we can see the skin of it from the outside as well as the foliage - if its knobbly, then Loosestrife's suggestion of Osage orange (Maclura pomifera) is likely correct

Hi all, researched other google pictures from your comments, and the leaves and 'fruit' confirm it is a walnut tree ! Thank you for putting us on the right track !

Well done. Lovely tree to have but you can't grow much under them because of their toxicity, but the nuts are tasty!