How to cook beef shank so that it is fall-off-the-bone tender? I bought several one-inch beef shank cross-sections from the local supermarket.
It looks like concentric circles, with a inner circle of bone and an innermost circle of marrow.
I put the shanks in a pot, covered it with water, and braised it for 12 hours at 200 F in the oven.
At the end, the collagen had converted into gelatin, but the meat seemed quite tough and chewy.

Is it possible for beef shanks to be super tender?
If so, how do you cook them that way?
If your beef shank is chewy, it's undercooked. Period. Whatever the amount of time is, even if it seems like a lot to you, it's not enough. Overcooking will cause meat cuts with a lot of connective tissue (like shank) to dry, and even fall apart because it is too tender. But it will not still be chewy.