Preheat oven to 350 degrees. Place the beets in a shallow roasting pan; add water until the beets are half submerged. Cover with aluminum foil and roast until the beets are tender when pierced ...
For the relish, quarter the cucumber lengthways, then thinly slice and place in a bowl along with the onion. Season with a few generous pinches of salt. Mix well and leave to rest for 15 minutes.