Step 1Place the yogurt in a small bowl. Fold a sheet of paper towel into quarters and place it directly on top of the yogurt to soak up the extra liquid. (This step isn't necessary if using Greek yogurt.) Set it aside.
Step 2Place the beets in a saucepan and add enough water to cover them completely. Add the salt and vinegar and cook over high heat until you can easily insert a fork into them, about 1 hour. Drain, then set them aside until they are cool enough to handle.
Step 3Peel the beets by hand (the skins should come off easily), using a paring knife if necessary. Place the beets in a food processor and pulse 3 to 5 times to make a coarsely chopped mixture. (Alternatively, use a potato masher or the back of a fork.)
Step 4Add the yogurt, garlic, lemon juice and sesame paste and stir to incorporate evenly. Taste and add more salt or lemon juice if desired. Garnish with the radish slices and serve cold or at room temperature. Makes 2 1/2 cups.