While chicken is cooking, place anchovy, garlic, and 1 tsp. salt in a mortar and use pestle to pound to a paste, about 1 minute. Add parsley and continue to pound until completely pulverized, about 2 minutes. (Alternately, grate garlic and finely chop anchovy and parsley, then mix together in a medium bowl.) Mix in oil, vinegar, mustard, and red pepper flakes with pestle, then add scallions. Taste and season with more salt, if needed.