How can I restore and keep a built-in cutting board in good condition?
I wouldn't use that board any further. Some of those gaps where the block laminate is separating go right the way through. I don't see how you could realistically sanitise it, even if the other side looks a bit better.
Not only would I replace it, I'd also seriously consider replacing the counter top. The chipboard is swollen due to repeated wetting &amp; is not going to be anything like hygienic any more. If you do consider only changing the chopping board, at least make sure the chipboard is properly sealed before you replace it, &amp; consider mounting the new board with silicone sealant around the edges.
Perhaps this may not apply world-wide, but in the UK that style of square-edged countertop hasn't been in use for at least 30 years. Pretty much everything since then is bullnose or semi-bullnose (half-round or quarter-round) - meaning there is no separate mica piece on the front edge, it's all made in one continuous, watertight, piece.
In consequence that chipboard could have more than 30 years of contamination in it.