My wife cut onions with a meaty knife on a milky chopping board. The board: one piece wood, never used for milky things other than onions, used within previous 24hrs. The knife: one piece metal, used for meat mamash but not within the previous 24hrs. The knife can obviously be kashered but can we (1) use the onions, and (2) use the board again?

What is milkey about a board used only for cutting onions? Unless you mean that you used a milky knife for cutting onions on that board in the past. In the future you should only use a parve cutting knife. The onions should be used only for meaty. The board is okay.