Blizzard have clarified that you can get the recipe for the Onyxia Scale Cloak (which has recently become relevant again due to being required for a toy treasure in Zaralekk Caverns) back if you’ve previously completed the quest but lost the recipe. The original question in the forum thread was framed for players that did have the pattern previously but since unlearned leatherworking and now got it back, so it doesn’t seem like you can get the pattern if you just learn(ed) leatherworking after having finished the Onyxia head quest, but that’s not quite clear.
Onyxia Scale Cloak (Source)
I think I have good news for you. If you’ve completed the quest to learn the Onyxia Scale Cloak, but lost the recipe, you can relearn it by speaking with Major Mattingly in Stormwind or Overlord Natoj in Orgrimmar.
