In a small bowl, stir together vinegar, water, salt + sugar until they dissolove. Add the julienned radish and combine. Using a 1.5" round cutter, cut 3 circles from each crumpet. Do the same with the havarti getting 5 circles from each slice. Melt butter in a non-stick skillet over medium heat. Toast the crumpets in butter, flat side up, then flip. Apply a little Dijon to the top of each, then a slice of havarti. Cover the skillet for a minute to help melt the havarti. Remove from the pan and top each with pickled radish.