To keep produce safe, a neat trick is to build your farm at a remote place you don't visit often. Since Valheim doesn't actively calculate anything in areas not visited by the player, the crops will stay completely unharmed. Wait for 3 ingame days after planting (remember onions take a day longer to grow!), then come back for harvest. You can even see the crops being updated from "freshly planted" to "fully grown" if you're fast enough.

You still might want to put a wooden fence around them to add a bit of safety when you're around for harvesting.