Whats the best method of securing floorboards to concrete?

there was an area of my kitchen floor that used to be a fireplace (concreete plinthe) so there was no floorboards in the area. I managed to remove the concrete only to be faced with further concrete 15mm lower. I have decided to place floorboards over this concrete to match the surrounding floor (stained and varnished floorboards) however was wondering what the best method of fixing these floorboards was. The area has very difficult access as some of it is under my kitchen units however i'm only interested in the area of floorboards that will show in front of the kickboard. These will be stained and varnished to match the existing floor.