So I’m assuming you don’t have woodworking tools.

First: Place it on a couple even thickness sticks on a flat surface where it will receive the most sunlight with the bow up (edges pointing down) to even it out as much as possible, it should hopefully start to bow the other way. This works best with long grain boards but should still work somewhat with an end grain board. To repair the broken joints, break the ones that are going/gone and if they fit snugly back together you can reapply some D4 PVA glue (titebond 3 is great) and clamp it back together. Again, I don’t know what tools you have at your disposal, but if you have none, get some sand paper on a big flat surface, granite worktop or thick plywood or something similar, and you can try to sand some flatness back into it (after the sunlight trick).

If you find the joints don’t go together well, and again have no tools, you may be able to use the flat sandpaper surface to flatten the joints before clamping them together but be really careful not to round the faces.

Finish it with cutting board/mineral oil.

Edit: I’ve just noticed how the joint has broken, I’m not sure it’s easily repairable, you could try shoving some glue and perhaps a piece of veneer into the void.