Boil the potatoes. In a medium pot, cover the potatoes with enough cold water so they’re comfortably submerged. Salt the water heavily (salty like the sea). Bring the potatoes to the boil, then turn the heat down to medium. A cake tester is the best way to assess doneness. Start testing after 10 minutes (but it may take up to 20 minutes, depending on the girth of your fingerlings). Drain and set aside to cool. Once cool, slice the potatoes into rounds.
Make the dressing. Whisk together all the ingredients. Taste for seasoning and adjust (be careful with the salt, as the miso is already salty).
Prep the brussels sprouts. Since we’re eating them raw, treat the brussels sprouts like little cabbages: trim the stem end liberally and peel off the dark green outer leaves. We want to get down to the light green inner leaves, removing any dark spots or weirdness. Cut each sprout in half and slice as thinly as you can.
Plate. In a mixing bowl, combine the potatoes, radicchio, brussels sprouts, and a few big spoonfuls of the dressing. Toss to coat and add more dressing if necessary. Mound a few spoonfuls of the salad on each plate and top with the chives, furikake, and bacon bits, if using.