This came up in one of my beds (northern Virginia zone 7a). I think it might be Cyrtomium fortunei, which is growing in another part of the yard -- or possibly Cyrtomium macrophyllum, but I don't think I have one of those, so have no idea where it could have come from. Does C. fortunei sound right? Doesn’t quite look like C. falcatum (which I also have growing elsewhere)which has wavier leaflet margins. Not sure how spores got over to this side of the house!

Sori arrangement on the underside of most mature fronds.