Wrap the beets in foil and place on a rack, set over a pan of hot water. Roast in a preheated 400 degree oven for 40 minutes. When cool enough to handle, peel, and slice into 1/4 inch disks.
Whisk the dry mustard into the buttermilk, then mix in the grain mustard, dill, onion and herring. Refrigerate the herring for 1 hour. Fan beets out on Bibb lettuce. Place herring mixture on top.