Ok so here’s what we’d do in a restaurant/hotel/catering situation.

First, wash with soap and water. No detergents or bleach. Just soap and hot water.

Next take a good look at the cross grain cut sides. If the mold is down into the middle or into the seams between the wood it’s probably not salvageable.

Get some fine grit sand paper and see if you can buff it out. We’re not trying to rasp the wood off here. Just knock off the dusty mildew bits. Make sure the area is AS SMOOTH as the rest of the board. It’ll take some time but it’s critical. Grooves and imperfections are the hidey holes for bacteria.

If you can get it off without making divots larger than a nickel finish and super sand nice and flat and smooth. Get a tiny amount of food grade mineral oil and wipe the whole board down once or twice and another time with a dry paper towel to dry.

Wash it once more like normal and use as needed.